diff --git a/applications/modelling_e2e/handler.py b/applications/modelling_e2e/handler.py
index d1f454f4..c2d11073 100644
--- a/applications/modelling_e2e/handler.py
+++ b/applications/modelling_e2e/handler.py
@@ -44,14 +44,23 @@ from domain.epc_prediction.comparable_properties import (
     select_comparables,
 )
 from domain.epc_prediction.epc_prediction import EpcPrediction
-from domain.epc_prediction.prediction_target import build_prediction_target
+from domain.epc_prediction.prediction_target import (
+    PredictionTarget,
+    build_prediction_target,
+)
 from domain.geospatial.coordinates import Coordinates
 from domain.geospatial.planning_restrictions import PlanningRestrictions
 from domain.geospatial.spatial_reference import SpatialReference
 from domain.property.property import Property, PropertyIdentity
+from domain.property_baseline.calculator_rebaseliner import CalculatorRebaseliner
+from domain.sap10_calculator.calculator import Sap10Calculator
 from domain.tasks.tasks import Source
 from harness.console import run_modelling
+from orchestration.property_baseline_orchestrator import (
+    PropertyBaselineOrchestrator,
+)
 from infrastructure.epc_client.epc_client_service import EpcClientService
+from infrastructure.postcodes_io.postcodes_io_client import PostcodesIoClient
 from infrastructure.postgres.config import PostgresConfig
 from infrastructure.postgres.engine import make_engine
 from infrastructure.solar.google_solar_api_client import (
@@ -69,6 +78,9 @@ from repositories.geospatial.geospatial_s3_repository import (
     GeospatialS3Repository,
     ParquetReader,
 )
+from repositories.fuel_rates.fuel_rates_static_file_repository import (
+    FuelRatesStaticFileRepository,
+)
 from repositories.postgres_unit_of_work import PostgresUnitOfWork
 from repositories.product.composite_product_repository import (
     catalogue_with_off_catalogue_overrides,
@@ -89,6 +101,10 @@ from utilities.logger import setup_logger
 
 _engine: Optional[Engine] = None
 _cohort_cache: dict[str, list[ComparableProperty]] = {}
+# Broadened (nearby-postcode) cohorts, keyed by (seed postcode, target property
+# type): the early-stop walk depends on the type it is filling for, so two types
+# in the same postcode must not share a cached result.
+_nearby_cohort_cache: dict[tuple[str, str], list[ComparableProperty]] = {}
 
 logger = setup_logger()
 
@@ -162,13 +178,18 @@ def _predict_epc(
     attributes_reader: OverrideBackedPredictionAttributesReader,
     coordinates: Optional[Coordinates],
     cohort_for: Callable[[str], list[ComparableProperty]],
+    broaden: Callable[[PredictionTarget], list[ComparableProperty]],
     predictor: EpcPrediction,
 ) -> Optional[EpcPropertyData]:
     """Synthesise an EpcPropertyData for an EPC-less property from its postcode
     cohort (EPC Prediction Path 3, ADR-0031), or None when ineligible.
 
+    When the property's own postcode holds no same-type comparables (a sparse
+    postcode — e.g. the only flat among houses), the cohort is broadened to the
+    real unit postcodes physically nearest it (``broaden``) before giving up.
+
     Returns None when property_type is unresolvable (hard cohort filter cannot
-    fire) or when the postcode cohort is empty after filtering.
+    fire) or when even the broadened cohort is empty after filtering.
     """
     attributes = attributes_reader.attributes_for(property_id)
     identity = PropertyIdentity(
@@ -178,6 +199,8 @@ def _predict_epc(
     if target is None:
         return None
     comparables = select_comparables(target, cohort_for(target.postcode))
+    if not comparables.members:
+        comparables = select_comparables(target, broaden(target))
     if not comparables.members:
         return None
     predicted = predictor.predict(target, comparables)
@@ -223,7 +246,9 @@ def handler(body: dict[str, Any], context: Any) -> Optional[dict[str, Any]]:
 
     overrides_reader = PropertyOverridesPostgresReader(lambda: Session(engine))
     prediction_attrs_reader = OverrideBackedPredictionAttributesReader(overrides_reader)
-    comparables_repo = EpcComparablePropertiesRepository(epc_client, geospatial)
+    comparables_repo = EpcComparablePropertiesRepository(
+        epc_client, geospatial, nearby_postcodes=PostcodesIoClient()
+    )
     predictor = EpcPrediction()
 
     def _get_cohort(postcode: str) -> list[ComparableProperty]:
@@ -233,6 +258,34 @@ def handler(body: dict[str, Any], context: Any) -> Optional[dict[str, Any]]:
             )
         return _cohort_cache[postcode]
 
+    def _broaden(target: PredictionTarget) -> list[ComparableProperty]:
+        """The nearby-postcode cohort for a gated-out target — the real unit
+        postcodes nearest it, walked until enough same-type comparables surface
+        (ADR-0034). Memoised per (postcode, property_type) so co-located
+        same-type misses share one walk."""
+        key = (target.postcode, target.property_type)
+        if key not in _nearby_cohort_cache:
+            _nearby_cohort_cache[key] = (
+                comparables_repo.candidates_near(
+                    target.postcode,
+                    target.coordinates,
+                    enough=lambda c: c.epc.property_type == target.property_type,
+                )
+                if target.postcode
+                else []
+            )
+        return _nearby_cohort_cache[key]
+
+    # Re-establishes each lodged Property's Baseline Performance from the just-
+    # persisted EPC (one UoW per property, committed after the Plan's). Predicted
+    # Properties have no lodged figures, so they get no baseline (mirrors the e2e
+    # runner and the ara_first_run Baseline stage).
+    baseline_orchestrator = PropertyBaselineOrchestrator(
+        unit_of_work=lambda: PostgresUnitOfWork(lambda: Session(engine)),
+        rebaseliner=CalculatorRebaseliner(Sap10Calculator()),
+        fuel_rates=FuelRatesStaticFileRepository(),
+    )
+
     read_session = Session(engine)
     try:
         scenario = ScenarioPostgresRepository(read_session).get_many([scenario_id])[0]
@@ -284,12 +337,14 @@ def handler(body: dict[str, Any], context: Any) -> Optional[dict[str, Any]]:
                         attributes_reader=prediction_attrs_reader,
                         coordinates=coordinates,
                         cohort_for=_get_cohort,
+                        broaden=_broaden,
                         predictor=predictor,
                     )
                     if predicted_epc is None:
                         raise ValueError(
                             f"no EPC for UPRN {uprn} and not predictable "
-                            f"(unresolved property_type or empty '{postcode}' cohort)"
+                            f"(unresolved property_type, or no same-type "
+                            f"comparables in or near '{postcode}')"
                         )
                     effective_epc = Property(
                         identity=PropertyIdentity(
@@ -375,6 +430,13 @@ def handler(body: dict[str, Any], context: Any) -> Optional[dict[str, Any]]:
                     uow.commit()
                 logger.info(f"property={property_id} plan saved")
 
+                # Baseline Performance is re-established from the persisted EPC, so
+                # it runs after the Plan UoW commits. Only lodged Properties have
+                # the lodged figures the Baseline reads; predicted ones are skipped.
+                if epc is not None:
+                    baseline_orchestrator.run([property_id])
+                    logger.info(f"property={property_id} baseline saved")
+
             except Exception as error:  # noqa: BLE001
                 logger.error(
                     f"property={property_id} uprn={uprns.get(property_id)}: "
diff --git a/docs/adr/0034-epc-prediction-cohort-broadens-to-nearby-postcodes.md b/docs/adr/0034-epc-prediction-cohort-broadens-to-nearby-postcodes.md
new file mode 100644
index 00000000..29d58d45
--- /dev/null
+++ b/docs/adr/0034-epc-prediction-cohort-broadens-to-nearby-postcodes.md
@@ -0,0 +1,90 @@
+# EPC Prediction cohort broadens to nearby postcodes
+
+ADR-0029 sizes an EPC-less Property from the **Comparable Properties** in its
+postcode; ADR-0031 decision 5 made `property_type` the **hard** cohort filter — a
+flat is never sized from houses — and gated out any Property with no same-type
+comparable. This records how we **broaden** the cohort when a Property's own
+postcode is too sparse to fill, rather than gating it out. Resolved against a
+real failure on branch `feature/e2e-runs`: property 718580, the only flat lodged
+in postcode `BR6 6BS` (the other 8 certs were houses/bungalows), gated out and
+failed its modelling subtask.
+
+## Status
+
+Accepted. Extends ADR-0029 / ADR-0031 decision 5 (the postcode cohort).
+
+## Context
+
+ADR-0031 decision 5 anticipated this — "an Ordnance Survey `postcode_search`
+source can supply property type more broadly … wiring it is a later enhancement
+that widens the eligible population." The gating it describes is correct (never
+size a flat from a mixed-type cohort), but it makes prediction only as good as
+the *own* postcode's lodged stock. A genuinely-isolated type (one flat among
+houses) is unpredictable with no recourse, and a no-EPC + can't-predict Property
+fails its subtask — which, over SQS, retries and dead-letters even though the
+input data is permanently insufficient.
+
+The legacy engine handled this by **trimming the postcode** (`BR6 6BS` → `BR6 6B`
+→ `BR6` …) and re-querying, because the old EPC API accepted partial postcodes.
+The current gov EPC API (`api.get-energy-performance-data.communities.gov.uk`)
+does **not**: its domestic search accepts only a *full* real postcode, `uprn`,
+`council[]`, `constituency[]`, or `address` — confirmed against its OpenAPI spec.
+There is no outcode/prefix, radius, or lat/long search.
+
+## Decisions
+
+### 1. Broaden to the real unit postcodes physically nearest the target
+
+When the own-postcode cohort yields no same-type member, the cohort is broadened
+to the real unit postcodes around the target, resolved from its coordinates via
+**postcodes.io**'s keyless `nearest` endpoint (already a trusted dependency in
+`scripts/fetch_epc_prediction_dense_corpus.py`). Each nearby postcode is then
+searched exactly as the own postcode is — so `property_type` stays the same hard
+filter and ADR-0031 decision 5 is upheld; only the *reach* of the candidate pool
+widens, never the selection rule. The hard gate still fires when even the
+broadened cohort has no same-type comparable: broadening widens the eligible
+population, it does not force a prediction.
+
+### 2. postcodes.io over `council[]` and a bulk centroid dataset
+
+- **`council[]`** (the only "wider than postcode" gov-API filter) returns the
+  *whole* local authority — tens of thousands of certs, many pages — and its
+  result rows carry **no `property_type`**, so narrowing to the target's type
+  would mean fetching every certificate. Rejected: cost is unbounded by distance
+  and dominated by per-cert fetches.
+- **OS Code-Point Open / ONSPD → S3** would give an offline unit-postcode→
+  coordinate table, but is a ~1.7M-row asset to ingest and host for a path that
+  fires rarely. Rejected as disproportionate; reconsider if postcodes.io's
+  availability becomes a liability.
+- **OS Places radius API** needs an `OS_API_KEY` the modelling path does not
+  otherwise carry. Rejected to avoid a new secret.
+
+postcodes.io is keyless, bounded by radius (a tight neighbourhood, not an
+authority), and reuses the existing per-postcode search unchanged.
+
+### 3. Lazy, nearest-first with early-stop, and soft-failing
+
+Broadening runs **only on gate-out** (a no-EPC Property whose own postcode lacks
+a same-type comparable) — the common cases never pay for it. Nearby postcodes are
+walked **nearest first** and the walk **stops early** once enough same-type
+comparables surface, so a dense area resolves in one or two searches instead of
+the whole radius. Results are memoised per `(postcode, property_type)`. The
+postcodes.io call is **soft-failing**: any error (after the shared transient
+retry) degrades to the seed postcode alone, so a flaky or unavailable
+postcodes.io can never break prediction — it just declines to broaden.
+
+## Consequences
+
+- A new external dependency, postcodes.io, sits in the modelling/Lambda path. It
+  is keyless and isolated behind `PostcodesIoClient` (shared transient-retry +
+  soft-fail), but its availability and rate limits are now a (bounded) operational
+  concern. ADR-0031 decision 5's `postcode_search` lever remains an alternative.
+- The `ComparablePropertiesRepository` gains `candidates_near` alongside
+  `candidates_for`; the predictor/handler try the own postcode first and broaden
+  only on an empty cohort.
+- Prediction reaches across postcode boundaries, so the geo-distance weighting
+  ADR-0029 already applies (a comparable's vote decays with haversine distance)
+  becomes load-bearing for broadened cohorts, not just a refinement.
+- `property_type` gating is unchanged: a Property with no same-type comparable
+  *anywhere nearby* is still gated out — now a genuine "no comparable stock"
+  signal rather than an artefact of a sparse single postcode.
diff --git a/infrastructure/epc_client/_retry.py b/infrastructure/epc_client/_retry.py
deleted file mode 100644
index e9f18dcb..00000000
--- a/infrastructure/epc_client/_retry.py
+++ /dev/null
@@ -1,35 +0,0 @@
-import time
-from typing import Callable, Optional, TypeVar
-
-import httpx
-
-from infrastructure.epc_client.exceptions import EpcRateLimitError
-
-T = TypeVar("T")
-
-
-def call_with_retry(
-    fn: Callable[[], T],
-    max_retries: int = 5,
-    backoff_base: float = 1.0,
-    backoff_multiplier: float = 2.0,
-    max_backoff: float = 60.0,
-) -> T:
-    """Retry `fn` on transient EPC-API failures: HTTP 429 rate limits and
-    transport errors (read/connect timeouts, connection resets). A 429 honours
-    the server's `Retry-After`; transport errors back off exponentially. Non-
-    transient failures (other 4xx/5xx, mapping errors) propagate immediately."""
-    last_exc: Optional[Exception] = None
-    for attempt in range(max_retries + 1):
-        try:
-            return fn()
-        except (EpcRateLimitError, httpx.TransportError) as exc:
-            last_exc = exc
-            if attempt < max_retries:
-                if isinstance(exc, EpcRateLimitError) and exc.retry_after is not None:
-                    delay = exc.retry_after
-                else:
-                    delay = backoff_base * (backoff_multiplier**attempt)
-                time.sleep(min(delay, max_backoff))
-    assert last_exc is not None
-    raise last_exc
diff --git a/infrastructure/epc_client/epc_client_service.py b/infrastructure/epc_client/epc_client_service.py
index 111de1f5..692b10a2 100644
--- a/infrastructure/epc_client/epc_client_service.py
+++ b/infrastructure/epc_client/epc_client_service.py
@@ -10,7 +10,7 @@ from infrastructure.epc_client.exceptions import (
     EpcNotFoundError,
     EpcRateLimitError,
 )
-from infrastructure.epc_client._retry import call_with_retry
+from infrastructure.http_retry import call_with_retry
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
 from datatypes.epc.domain.mapper import EpcPropertyDataMapper
 from datatypes.epc.search import EpcSearchResult
diff --git a/infrastructure/epc_client/exceptions.py b/infrastructure/epc_client/exceptions.py
index fb7d96fa..af28fb74 100644
--- a/infrastructure/epc_client/exceptions.py
+++ b/infrastructure/epc_client/exceptions.py
@@ -1,4 +1,4 @@
-from typing import Optional
+from infrastructure.http_retry import TransientHttpError
 
 
 class EpcApiError(Exception):
@@ -9,9 +9,7 @@ class EpcNotFoundError(EpcApiError):
     """Raised when the API returns 404."""
 
 
-class EpcRateLimitError(EpcApiError):
-    """Raised when the API returns 429 and all retries are exhausted."""
-
-    def __init__(self, message: str, retry_after: Optional[float] = None) -> None:
-        super().__init__(message)
-        self.retry_after = retry_after
+class EpcRateLimitError(EpcApiError, TransientHttpError):
+    """Raised when the API returns 429. A ``TransientHttpError`` so the shared
+    ``call_with_retry`` retries it (honouring ``Retry-After``), while remaining an
+    ``EpcApiError`` for callers that catch the EPC hierarchy."""
diff --git a/infrastructure/http_retry.py b/infrastructure/http_retry.py
new file mode 100644
index 00000000..aa3689a5
--- /dev/null
+++ b/infrastructure/http_retry.py
@@ -0,0 +1,60 @@
+"""Shared transient-failure retry for the HTTP source clients.
+
+The retry *mechanism* is generic; each client owns the *policy* of what counts as
+transient. A caller signals "retry this" by raising ``TransientHttpError``
+(carrying any server-advised ``retry_after``); transport-level errors (read /
+connect timeouts, connection resets) are always treated as transient.
+``call_with_retry`` backs off exponentially between attempts — honouring
+``retry_after`` when present — and re-raises the last error once attempts are
+exhausted, leaving the caller to decide how to surface it (the EPC client lets it
+propagate; postcodes.io soft-fails to the seed postcode).
+"""
+
+from __future__ import annotations
+
+import time
+from typing import Callable, Optional, TypeVar
+
+import httpx
+
+T = TypeVar("T")
+
+
+class TransientHttpError(Exception):
+    """A failure worth retrying. ``retry_after`` is the server-advised delay (a
+    429's ``Retry-After``), used in place of the computed backoff when present."""
+
+    def __init__(self, message: str, retry_after: Optional[float] = None) -> None:
+        super().__init__(message)
+        self.retry_after = retry_after
+
+
+def call_with_retry(
+    fn: Callable[[], T],
+    max_retries: int = 5,
+    backoff_base: float = 1.0,
+    backoff_multiplier: float = 2.0,
+    max_backoff: float = 60.0,
+) -> T:
+    """Retry ``fn`` on transient failures — ``TransientHttpError`` (e.g. a 429)
+    and ``httpx.TransportError`` (read/connect timeouts, connection resets) —
+    backing off exponentially, or by the error's ``retry_after`` when it carries
+    one. Non-transient failures propagate immediately; the last transient error
+    is re-raised once ``max_retries`` is exhausted."""
+    last_exc: Optional[Exception] = None
+    for attempt in range(max_retries + 1):
+        try:
+            return fn()
+        except (TransientHttpError, httpx.TransportError) as exc:
+            last_exc = exc
+            if attempt < max_retries:
+                retry_after = (
+                    exc.retry_after if isinstance(exc, TransientHttpError) else None
+                )
+                if retry_after is not None:
+                    delay = retry_after
+                else:
+                    delay = backoff_base * (backoff_multiplier**attempt)
+                time.sleep(min(delay, max_backoff))
+    assert last_exc is not None
+    raise last_exc
diff --git a/infrastructure/postcodes_io/__init__.py b/infrastructure/postcodes_io/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/infrastructure/postcodes_io/postcodes_io_client.py b/infrastructure/postcodes_io/postcodes_io_client.py
new file mode 100644
index 00000000..7216d1b1
--- /dev/null
+++ b/infrastructure/postcodes_io/postcodes_io_client.py
@@ -0,0 +1,145 @@
+"""postcodes.io adapter — a coordinate (or seed postcode) → the real unit
+postcodes physically near it.
+
+The gov EPC API only searches a *full* real postcode — no outcode/prefix, no
+radius, no lat/long (confirmed against its OpenAPI spec). So to broaden an
+EPC-Prediction cohort beyond the target's own postcode we must first discover the
+real unit postcodes around it. postcodes.io's free, keyless ``nearest`` endpoint
+does exactly that: given a point it returns the unit postcodes within a radius,
+nearest first.
+
+Failure is deliberately non-fatal: any error (network, unknown seed, missing
+coordinates) returns just the seed postcode, so broadening degrades to "no
+broadening" rather than breaking prediction.
+"""
+
+from __future__ import annotations
+
+from typing import Any, Optional
+
+import httpx
+
+from domain.geospatial.coordinates import Coordinates
+from infrastructure.http_retry import TransientHttpError, call_with_retry
+
+
+class PostcodesIoClient:
+    BASE_URL = "https://api.postcodes.io"
+    REQUEST_TIMEOUT = 10.0
+    # Transient failures (transport errors, 429s, 5xx) are retried with
+    # exponential backoff; everything else (and exhaustion) soft-fails to the
+    # seed, so broadening never breaks prediction.
+    MAX_RETRIES = 3
+    BACKOFF_BASE = 0.5
+    BACKOFF_MULTIPLIER = 2.0
+    MAX_BACKOFF = 8.0
+
+    def __init__(self, *, radius_m: int = 1000, limit: int = 30) -> None:
+        """``radius_m`` bounds how far the broadened cohort reaches; ``limit``
+        caps how many nearby postcodes are returned (and so the per-gate-out
+        fetch cost)."""
+        self._radius_m = radius_m
+        self._limit = limit
+
+    def nearby(
+        self, postcode: str, coordinates: Optional[Coordinates] = None
+    ) -> list[str]:
+        """The real unit postcodes within ``radius_m`` of ``postcode`` — nearest
+        first, the seed always included — or just ``[postcode]`` when the seed's
+        coordinates cannot be resolved or the lookup fails.
+
+        ``coordinates`` (the target's own, resolved from its UPRN) is used when
+        given, sparing a postcode→centroid round-trip; otherwise postcodes.io
+        resolves the seed postcode's centroid itself."""
+        point = coordinates if coordinates is not None else self._centroid_of(postcode)
+        if point is None:
+            return [postcode]
+        found = self._nearest_to(point)
+        ordered = [postcode] + [p for p in found if p != postcode]
+        return ordered[: self._limit]
+
+    def _centroid_of(self, postcode: str) -> Optional[Coordinates]:
+        result = self._get(f"/postcodes/{postcode.replace(' ', '')}")
+        if result is None:
+            return None
+        latitude: Any = result.get("latitude")
+        longitude: Any = result.get("longitude")
+        if latitude is None or longitude is None:
+            return None
+        return Coordinates(longitude=float(longitude), latitude=float(latitude))
+
+    def _nearest_to(self, point: Coordinates) -> list[str]:
+        results = self._get_list(
+            "/postcodes",
+            {
+                "lon": point.longitude,
+                "lat": point.latitude,
+                "radius": self._radius_m,
+                "limit": self._limit,
+            },
+        )
+        return [str(row["postcode"]) for row in results if row.get("postcode")]
+
+    def _get(self, path: str) -> Optional[dict[str, Any]]:
+        payload = self._call(path, None)
+        return payload if isinstance(payload, dict) else None
+
+    def _get_list(self, path: str, params: dict[str, Any]) -> list[dict[str, Any]]:
+        payload = self._call(path, params)
+        if not isinstance(payload, list):
+            return []
+        return [row for row in payload if isinstance(row, dict)]
+
+    def _call(self, path: str, params: Optional[dict[str, Any]]) -> Any:
+        """The parsed ``result`` payload for a postcodes.io GET, retried on
+        transient failures via the shared ``call_with_retry``, or None on a
+        non-transient failure (e.g. an unknown postcode's 404) or once retries are
+        exhausted — broadening then falls back to the seed alone. The soft-fail is
+        the difference from the EPC client, which lets the error propagate."""
+        try:
+            return call_with_retry(
+                lambda: self._fetch(path, params),
+                max_retries=self.MAX_RETRIES,
+                backoff_base=self.BACKOFF_BASE,
+                backoff_multiplier=self.BACKOFF_MULTIPLIER,
+                max_backoff=self.MAX_BACKOFF,
+            )
+        except (TransientHttpError, httpx.HTTPError):
+            return None
+
+    def _fetch(self, path: str, params: Optional[dict[str, Any]]) -> Any:
+        """One GET. Raises ``TransientHttpError`` on a 429/5xx (and lets
+        ``httpx.TransportError`` propagate) so ``call_with_retry`` retries it;
+        returns None for a non-transient non-success (e.g. 404) or unparseable
+        body."""
+        response = httpx.get(
+            f"{self.BASE_URL}{path}",
+            params=params,
+            timeout=self.REQUEST_TIMEOUT,
+        )
+        if self._is_transient(response.status_code):
+            raise TransientHttpError(
+                f"postcodes.io {response.status_code} on {path}",
+                retry_after=self._retry_after(response),
+            )
+        if not response.is_success:
+            return None
+        try:
+            body: Any = response.json()
+        except ValueError:
+            return None
+        return body.get("result") if isinstance(body, dict) else None
+
+    @staticmethod
+    def _is_transient(status_code: int) -> bool:
+        return status_code == 429 or status_code >= 500
+
+    @staticmethod
+    def _retry_after(response: httpx.Response) -> Optional[float]:
+        header = response.headers.get("Retry-After")
+        if header is None:
+            return None
+        try:
+            return float(header)
+        except (TypeError, ValueError):
+            return None
diff --git a/repositories/comparable_properties/epc_comparable_properties_repository.py b/repositories/comparable_properties/epc_comparable_properties_repository.py
index e1e7bd4d..20c51dfd 100644
--- a/repositories/comparable_properties/epc_comparable_properties_repository.py
+++ b/repositories/comparable_properties/epc_comparable_properties_repository.py
@@ -12,7 +12,7 @@ from __future__ import annotations
 import logging
 from dataclasses import dataclass
 from datetime import date
-from typing import Optional, Protocol
+from typing import Callable, Optional, Protocol
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
 from datatypes.epc.search.epc_search_result import EpcSearchResult
@@ -22,6 +22,11 @@ from repositories.comparable_properties.comparable_properties_repository import
     ComparablePropertiesRepository,
 )
 
+# The same default floor `select_comparables` uses: keep walking nearby postcodes
+# until this many candidates match, so the broadened cohort is big enough for the
+# downstream relax ladder rather than stopping at the first stray match.
+_DEFAULT_MINIMUM_COHORT = 5
+
 
 class CohortEpcClient(Protocol):
     """The slice of the EPC-API client the cohort fetch needs (e.g.
@@ -40,6 +45,16 @@ class CohortGeospatial(Protocol):
     ) -> dict[int, Coordinates]: ...
 
 
+class NearbyPostcodes(Protocol):
+    """Resolves the real unit postcodes physically near a seed postcode (e.g.
+    `PostcodesIoClient`). The gov EPC API cannot search by radius, so this is how
+    the cohort reaches beyond the target's own postcode (ADR-0034)."""
+
+    def nearby(
+        self, postcode: str, coordinates: Optional[Coordinates] = None
+    ) -> list[str]: ...
+
+
 logger = logging.getLogger(__name__)
 
 
@@ -56,10 +71,14 @@ class SkippedCohortCert:
 
 class EpcComparablePropertiesRepository(ComparablePropertiesRepository):
     def __init__(
-        self, epc_client: CohortEpcClient, geospatial: CohortGeospatial
+        self,
+        epc_client: CohortEpcClient,
+        geospatial: CohortGeospatial,
+        nearby_postcodes: Optional[NearbyPostcodes] = None,
     ) -> None:
         self._epc_client = epc_client
         self._geospatial = geospatial
+        self._nearby_postcodes = nearby_postcodes
         # Cohort certs skipped because they are not yet mappable. Accumulates
         # across every postcode the instance serves; the caller reads it after
         # the run to report the mapper gaps (see modelling_e2e handler).
@@ -80,6 +99,45 @@ class EpcComparablePropertiesRepository(ComparablePropertiesRepository):
                 cohort.append(comparable)
         return cohort
 
+    def candidates_near(
+        self,
+        postcode: str,
+        coordinates: Optional[Coordinates] = None,
+        *,
+        enough: Optional[Callable[[ComparableProperty], bool]] = None,
+        minimum: int = _DEFAULT_MINIMUM_COHORT,
+    ) -> list[ComparableProperty]:
+        """The broadened cohort: candidates drawn from the real unit postcodes
+        nearest ``postcode`` (ADR-0034), for when the target's own postcode holds
+        no same-type comparables. Postcodes are visited nearest first and each
+        candidate is deduped by certificate number across them.
+
+        ``enough`` lets the caller stop the walk early — once ``minimum``
+        candidates satisfy it (e.g. they match the target's property type) the
+        remaining, further-away postcodes are not fetched, so a dense area
+        resolves in one or two searches instead of the whole radius. Without a
+        configured ``NearbyPostcodes`` source this degrades to the seed postcode
+        alone."""
+        postcodes = (
+            self._nearby_postcodes.nearby(postcode, coordinates)
+            if self._nearby_postcodes is not None
+            else [postcode]
+        )
+        candidates: list[ComparableProperty] = []
+        seen_certs: set[str] = set()
+        matches = 0
+        for nearby_postcode in postcodes:
+            for candidate in self.candidates_for(nearby_postcode):
+                if candidate.certificate_number in seen_certs:
+                    continue
+                seen_certs.add(candidate.certificate_number)
+                candidates.append(candidate)
+                if enough is not None and enough(candidate):
+                    matches += 1
+            if enough is not None and matches >= minimum:
+                break
+        return candidates
+
     # Mapper-shape errors: the cert's lodged data does not fit the schema/mapper.
     # `ValueError` — missing required field / unmapped code (`UnmappedApiCode`);
     # `AttributeError` — a field lodged in the wrong shape (e.g. `photovoltaic_supply`
diff --git a/scripts/run_modelling_e2e.py b/scripts/run_modelling_e2e.py
index 6625aa49..1a1964ce 100644
--- a/scripts/run_modelling_e2e.py
+++ b/scripts/run_modelling_e2e.py
@@ -81,6 +81,7 @@ from domain.epc_prediction.comparable_properties import (  # noqa: E402
 )
 from domain.epc_prediction.epc_prediction import EpcPrediction  # noqa: E402
 from domain.epc_prediction.prediction_target import (  # noqa: E402
+    PredictionTarget,
     build_prediction_target,
 )
 from domain.geospatial.coordinates import Coordinates  # noqa: E402
@@ -96,6 +97,9 @@ from domain.modelling.scenario import Scenario  # noqa: E402
 from harness.console import candidate_recommendations, run_modelling  # noqa: E402
 from harness.plan_table import format_plan_table  # noqa: E402
 from infrastructure.epc_client.epc_client_service import EpcClientService  # noqa: E402
+from infrastructure.postcodes_io.postcodes_io_client import (  # noqa: E402
+    PostcodesIoClient,
+)
 from infrastructure.solar.google_solar_api_client import (  # noqa: E402
     BuildingInsightsNotFoundError,
     GoogleSolarApiClient,
@@ -400,6 +404,7 @@ def _predict_epc(
     attributes_reader: OverrideBackedPredictionAttributesReader,
     coordinates: Optional[Coordinates],
     cohort_for: Callable[[str], list[ComparableProperty]],
+    broaden: Callable[[PredictionTarget], list[ComparableProperty]],
     predictor: EpcPrediction,
 ) -> Optional[EpcPropertyData]:
     """Synthesise an EpcPropertyData for an EPC-less Property from its postcode
@@ -408,7 +413,8 @@ def _predict_epc(
 
     The cohort is found by POSTCODE, so a wrong postcode on the property row
     yields the wrong neighbours — a prediction is only as good as the postcode it
-    is given."""
+    is given. When the own postcode holds no same-type comparables, the cohort is
+    broadened to the real unit postcodes physically nearest it (``broaden``)."""
     attributes = attributes_reader.attributes_for(property_id)
     identity = PropertyIdentity(
         portfolio_id=portfolio_id, postcode=postcode, address="", uprn=uprn
@@ -418,7 +424,10 @@ def _predict_epc(
         return None  # property_type unresolvable — gated out of prediction
     comparables = select_comparables(target, cohort_for(target.postcode))
     if not comparables.members:
-        return None  # no comparable neighbours in the postcode
+        # Sparse own postcode — reach out to the nearest real postcodes.
+        comparables = select_comparables(target, broaden(target))
+    if not comparables.members:
+        return None  # no comparable neighbours nearby either
     predicted = predictor.predict(target, comparables)
     # The calculator needs a MAIN building part; a cohort whose template carries
     # none (e.g. a malformed flat record) yields an unscoreable picture, so reject
@@ -684,9 +693,12 @@ def main() -> None:
     # from the live EPC API (search-by-postcode + per-cert fetch), memoised per
     # postcode so co-located missing Properties don't refetch the same cohort.
     prediction_attributes = OverrideBackedPredictionAttributesReader(overrides_reader)
-    comparables_repo = EpcComparablePropertiesRepository(epc_client, geospatial)
+    comparables_repo = EpcComparablePropertiesRepository(
+        epc_client, geospatial, nearby_postcodes=PostcodesIoClient()
+    )
     predictor = EpcPrediction()
     _cohort_cache: dict[str, list[ComparableProperty]] = {}
+    _nearby_cohort_cache: dict[tuple[str, str], list[ComparableProperty]] = {}
 
     def cohort_for(postcode: str) -> list[ComparableProperty]:
         if postcode not in _cohort_cache:
@@ -694,6 +706,23 @@ def main() -> None:
                 comparables_repo.candidates_for(postcode) if postcode else []
             )
         return _cohort_cache[postcode]
+
+    def broaden(target: PredictionTarget) -> list[ComparableProperty]:
+        # Broadened cohort for a gated-out target: the nearest real postcodes,
+        # walked until enough same-type comparables surface (ADR-0034). Memoised
+        # per (postcode, property_type).
+        key = (target.postcode, target.property_type)
+        if key not in _nearby_cohort_cache:
+            _nearby_cohort_cache[key] = (
+                comparables_repo.candidates_near(
+                    target.postcode,
+                    target.coordinates,
+                    enough=lambda c: c.epc.property_type == target.property_type,
+                )
+                if target.postcode
+                else []
+            )
+        return _nearby_cohort_cache[key]
     # One read-only session for the live `material` catalogue, reused across the
     # batch so both store and no-store runs price against the same DB rows.
     catalogue_session = Session(engine)
@@ -831,12 +860,14 @@ def main() -> None:
                     attributes_reader=prediction_attributes,
                     coordinates=coordinates,
                     cohort_for=cohort_for,
+                    broaden=broaden,
                     predictor=predictor,
                 )
                 if predicted_epc is None:
                     raise ValueError(
                         f"no EPC for UPRN {uprn} and not predictable "
-                        f"(unresolved property_type or empty '{postcode}' cohort)"
+                        f"(unresolved property_type, or no same-type "
+                        f"comparables in or near '{postcode}')"
                     )
                 # Property.effective_epc folds any Landlord Overrides onto the
                 # synthesised EPC (cohort fills the unknown fields, the landlord's
diff --git a/tests/applications/modelling_e2e/test_handler.py b/tests/applications/modelling_e2e/test_handler.py
index e7ac4ced..6929d13f 100644
--- a/tests/applications/modelling_e2e/test_handler.py
+++ b/tests/applications/modelling_e2e/test_handler.py
@@ -8,7 +8,7 @@ is needed. One test per distinct behaviour path.
 from __future__ import annotations
 
 from contextlib import ExitStack
-from typing import Any
+from typing import Any, Iterator
 from unittest.mock import MagicMock, call, patch
 
 import pytest
@@ -81,6 +81,17 @@ def _clear_cohort_cache() -> None:
     import applications.modelling_e2e.handler as h
 
     h._cohort_cache.clear()
+    h._nearby_cohort_cache.clear()
+
+
+@pytest.fixture(autouse=True)
+def _baseline_orchestrator() -> Iterator[MagicMock]:
+    """Patch the Baseline orchestrator for every test — construction stays cheap
+    and ``.run`` is a no-op mock the baseline tests assert against."""
+    with patch(
+        "applications.modelling_e2e.handler.PropertyBaselineOrchestrator"
+    ) as orchestrator:
+        yield orchestrator
 
 
 # ---------------------------------------------------------------------------
@@ -116,9 +127,12 @@ def test_trigger_body_rejects_missing_property_ids() -> None:
 # ---------------------------------------------------------------------------
 
 
-def test_lodged_epc_path_saves_epc_plan_and_marks_modelled() -> None:
+def test_lodged_epc_path_saves_epc_plan_and_marks_modelled(
+    _baseline_orchestrator: MagicMock,
+) -> None:
     """When get_by_uprn returns an EPC the handler saves it, saves the plan,
-    and marks the property as modelled — all inside one UoW per property."""
+    marks the property as modelled — all inside one UoW per property — and
+    re-establishes its Baseline Performance after the plan commits."""
     # Arrange
     mock_engine = _engine_mock([PROPERTY_ID], [UPRN], [POSTCODE])
     mock_epc = MagicMock()
@@ -196,6 +210,8 @@ def test_lodged_epc_path_saves_epc_plan_and_marks_modelled() -> None:
         PROPERTY_ID, has_recommendations=False
     )
     mock_uow.commit.assert_called_once()
+    # Baseline Performance is re-established for the lodged property.
+    _baseline_orchestrator.return_value.run.assert_called_once_with([PROPERTY_ID])
 
 
 def test_skipped_cohort_certs_are_surfaced_in_the_outputs() -> None:
@@ -296,9 +312,12 @@ def test_skipped_cohort_certs_are_surfaced_in_the_outputs() -> None:
 # ---------------------------------------------------------------------------
 
 
-def test_prediction_path_saves_plan_without_epc_save() -> None:
+def test_prediction_path_saves_plan_without_epc_save(
+    _baseline_orchestrator: MagicMock,
+) -> None:
     """When get_by_uprn returns None the handler synthesises an EPC via
-    prediction and saves the plan — but never calls epc.save."""
+    prediction and saves the plan — but never calls epc.save, and (having no
+    lodged figures) never establishes a Baseline."""
     # Arrange
     mock_engine = _engine_mock([PROPERTY_ID], [UPRN], [POSTCODE])
     mock_plan = _plan_mock()
@@ -398,10 +417,11 @@ def test_prediction_path_saves_plan_without_epc_save() -> None:
         # Act
         _call_handler(_BODY)
 
-    # Assert — epc.save NOT called (no lodged cert), plan IS saved
+    # Assert — epc.save NOT called (no lodged cert), plan IS saved, no baseline
     mock_uow.epc.save.assert_not_called()
     mock_uow.plan.save.assert_called_once()
     mock_uow.commit.assert_called_once()
+    _baseline_orchestrator.return_value.run.assert_not_called()
 
 
 # ---------------------------------------------------------------------------
@@ -489,6 +509,124 @@ def test_empty_cohort_gates_property_out_and_raises() -> None:
     MockUoW.return_value.__enter__.assert_not_called()
 
 
+# ---------------------------------------------------------------------------
+# Broadened cohort — sparse own postcode falls back to nearby postcodes
+# ---------------------------------------------------------------------------
+
+
+def test_empty_own_postcode_broadens_to_nearby_and_predicts() -> None:
+    """When the property's own postcode holds no same-type comparables, the
+    handler broadens to the nearby-postcode cohort (candidates_near) and, finding
+    comparables there, synthesises the EPC and saves the plan."""
+    # Arrange
+    mock_engine = _engine_mock([PROPERTY_ID], [UPRN], [POSTCODE])
+    mock_plan = _plan_mock()
+    mock_uow = MagicMock()
+
+    mock_predicted_epc = MagicMock()
+    from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+
+    mock_part = MagicMock()
+    mock_part.identifier = BuildingPartIdentifier.MAIN
+    mock_predicted_epc.sap_building_parts = [mock_part]
+
+    # First select_comparables (own postcode) is empty → broaden; the second
+    # (nearby cohort) finds comparables.
+    empty_comparables = MagicMock()
+    empty_comparables.members = []
+    found_comparables = MagicMock()
+    found_comparables.members = [MagicMock()]
+
+    with ExitStack() as stack:
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.os.environ", _ENV)
+        )
+        stack.enter_context(
+            patch(
+                "applications.modelling_e2e.handler._get_engine",
+                return_value=mock_engine,
+            )
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.EpcClientService")
+        ).return_value.get_by_uprn.return_value = None  # no lodged EPC
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.GeospatialS3Repository")
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.GoogleSolarApiClient")
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler._spatial_for", return_value=None)
+        )
+        stack.enter_context(
+            patch(
+                "applications.modelling_e2e.handler._solar_insights_for",
+                return_value=None,
+            )
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.overlays_from", return_value=[])
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.PropertyOverridesPostgresReader")
+        )
+        from domain.epc_prediction.prediction_target import PredictionTargetAttributes
+
+        stack.enter_context(
+            patch(
+                "applications.modelling_e2e.handler.OverrideBackedPredictionAttributesReader"
+            )
+        ).return_value.attributes_for.return_value = PredictionTargetAttributes(
+            property_type="2"
+        )
+        MockRepo = stack.enter_context(
+            patch(
+                "applications.modelling_e2e.handler.EpcComparablePropertiesRepository"
+            )
+        )
+        MockRepo.return_value.candidates_for.return_value = []
+        MockRepo.return_value.candidates_near.return_value = [MagicMock()]
+        stack.enter_context(
+            patch(
+                "applications.modelling_e2e.handler.select_comparables",
+                side_effect=[empty_comparables, found_comparables],
+            )
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.EpcPrediction")
+        ).return_value.predict.return_value = mock_predicted_epc
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.ScenarioPostgresRepository")
+        ).return_value.get_many.return_value = [MagicMock()]
+        stack.enter_context(
+            patch(
+                "applications.modelling_e2e.handler.catalogue_with_off_catalogue_overrides"
+            )
+        )
+        stack.enter_context(patch("applications.modelling_e2e.handler.Session"))
+        stack.enter_context(
+            patch(
+                "applications.modelling_e2e.handler.run_modelling",
+                return_value=mock_plan,
+            )
+        )
+        MockUoW = stack.enter_context(
+            patch("applications.modelling_e2e.handler.PostgresUnitOfWork")
+        )
+        MockUoW.return_value.__enter__.return_value = mock_uow
+        MockUoW.return_value.__exit__.return_value = False
+
+        # Act
+        _call_handler(_BODY)
+
+    # Assert — broadening fired, and the broadened cohort produced a saved plan.
+    MockRepo.return_value.candidates_near.assert_called_once()
+    mock_uow.epc.save.assert_not_called()  # predicted, never lodged
+    mock_uow.plan.save.assert_called_once()
+    mock_uow.commit.assert_called_once()
+
+
 # ---------------------------------------------------------------------------
 # Partial batch failure
 # ---------------------------------------------------------------------------
diff --git a/tests/infrastructure/epc_client/test_client.py b/tests/infrastructure/epc_client/test_client.py
index 190513ce..97a0403b 100644
--- a/tests/infrastructure/epc_client/test_client.py
+++ b/tests/infrastructure/epc_client/test_client.py
@@ -78,7 +78,7 @@ def test_429_retry_after_header_drives_sleep_duration(
         _mock_response(200, cert_response),
     ]
     with patch("httpx.get", side_effect=responses), patch(
-        "infrastructure.epc_client._retry.time.sleep"
+        "infrastructure.http_retry.time.sleep"
     ) as mock_sleep:
         epc_service.get_by_certificate_number("CERT-001")
 
@@ -100,7 +100,7 @@ def test_429_without_retry_after_uses_exponential_backoff(
         _mock_response(200, cert_response),
     ]
     with patch("httpx.get", side_effect=responses), patch(
-        "infrastructure.epc_client._retry.time.sleep"
+        "infrastructure.http_retry.time.sleep"
     ) as mock_sleep:
         epc_service.get_by_certificate_number("CERT-001")
 
@@ -121,7 +121,7 @@ def test_429_malformed_retry_after_falls_back_to_backoff(
         _mock_response(200, cert_response),
     ]
     with patch("httpx.get", side_effect=responses), patch(
-        "infrastructure.epc_client._retry.time.sleep"
+        "infrastructure.http_retry.time.sleep"
     ) as mock_sleep:
         epc_service.get_by_certificate_number("CERT-001")
 
@@ -140,7 +140,7 @@ def test_429_retry_after_capped_by_max_backoff(epc_service, rdsap_21_0_1_cert):
         _mock_response(200, cert_response),
     ]
     with patch("httpx.get", side_effect=responses), patch(
-        "infrastructure.epc_client._retry.time.sleep"
+        "infrastructure.http_retry.time.sleep"
     ) as mock_sleep:
         epc_service.get_by_certificate_number("CERT-001")
 
diff --git a/tests/infrastructure/postcodes_io/__init__.py b/tests/infrastructure/postcodes_io/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/infrastructure/postcodes_io/test_postcodes_io_client.py b/tests/infrastructure/postcodes_io/test_postcodes_io_client.py
new file mode 100644
index 00000000..4d5e34a0
--- /dev/null
+++ b/tests/infrastructure/postcodes_io/test_postcodes_io_client.py
@@ -0,0 +1,224 @@
+"""PostcodesIoClient — coordinate/seed postcode → the real unit postcodes near
+it, via postcodes.io's keyless nearest endpoint. Failure degrades to the seed
+alone so broadening never breaks prediction."""
+
+from __future__ import annotations
+
+from typing import Any, Iterator, Optional
+from unittest.mock import MagicMock, patch
+
+import httpx
+import pytest
+
+from domain.geospatial.coordinates import Coordinates
+from infrastructure.postcodes_io.postcodes_io_client import PostcodesIoClient
+
+_MODULE = "infrastructure.postcodes_io.postcodes_io_client"
+
+
+@pytest.fixture(autouse=True)
+def _no_sleep() -> Iterator[MagicMock]:
+    """Never actually sleep during backoff (the shared retry owns the sleep) —
+    just record the calls."""
+    with patch("infrastructure.http_retry.time.sleep") as sleep:
+        yield sleep
+
+
+def _response(
+    payload: Any,
+    *,
+    status_code: int = 200,
+    headers: Optional[dict[str, str]] = None,
+) -> MagicMock:
+    resp = MagicMock()
+    resp.status_code = status_code
+    resp.is_success = 200 <= status_code < 300
+    resp.headers = headers if headers is not None else {}
+    resp.json.return_value = payload
+    return resp
+
+
+def _nearest_payload(postcodes: list[str]) -> dict[str, Any]:
+    return {"result": [{"postcode": p} for p in postcodes]}
+
+
+def test_nearby_with_coordinates_skips_the_centroid_lookup() -> None:
+    """When the target's own coordinates are passed, only the radius search is
+    issued — no postcode→centroid round-trip — and the seed leads the result."""
+    # Arrange
+    client = PostcodesIoClient(radius_m=500, limit=10)
+    coords = Coordinates(longitude=0.1, latitude=51.3)
+
+    with patch(f"{_MODULE}.httpx.get") as mock_get:
+        mock_get.return_value = _response(
+            _nearest_payload(["BR6 6BS", "BR6 6BU", "BR6 6NX"])
+        )
+
+        # Act
+        result = client.nearby("BR6 6BS", coords)
+
+    # Assert — one call (the radius search), seed first, neighbours follow
+    assert result == ["BR6 6BS", "BR6 6BU", "BR6 6NX"]
+    assert mock_get.call_count == 1
+    _, kwargs = mock_get.call_args
+    assert kwargs["params"]["lat"] == 51.3
+    assert kwargs["params"]["lon"] == 0.1
+
+
+def test_nearby_resolves_the_seed_centroid_when_no_coordinates_given() -> None:
+    """Without coordinates the client first resolves the seed's own centroid via
+    postcodes.io, then runs the radius search from it."""
+    # Arrange
+    client = PostcodesIoClient()
+    centroid = {"result": {"latitude": 51.3, "longitude": 0.1}}
+
+    with patch(f"{_MODULE}.httpx.get") as mock_get:
+        mock_get.side_effect = [
+            _response(centroid),
+            _response(_nearest_payload(["BR6 6BS", "BR6 6BU"])),
+        ]
+
+        # Act
+        result = client.nearby("BR6 6BS")
+
+    # Assert — two calls: centroid then radius
+    assert result == ["BR6 6BS", "BR6 6BU"]
+    assert mock_get.call_count == 2
+
+
+def test_nearby_dedupes_the_seed_and_caps_at_limit() -> None:
+    """The seed always leads exactly once even when the radius search echoes it,
+    and the result is capped at ``limit``."""
+    # Arrange
+    client = PostcodesIoClient(limit=3)
+    coords = Coordinates(longitude=0.1, latitude=51.3)
+
+    with patch(f"{_MODULE}.httpx.get") as mock_get:
+        mock_get.return_value = _response(
+            _nearest_payload(["BR6 6BS", "BR6 6BU", "BR6 6NX", "BR6 6AA"])
+        )
+
+        # Act
+        result = client.nearby("BR6 6BS", coords)
+
+    # Assert
+    assert result == ["BR6 6BS", "BR6 6BU", "BR6 6NX"]
+    assert result.count("BR6 6BS") == 1
+
+
+def test_nearby_returns_just_the_seed_after_exhausting_retries(
+    _no_sleep: MagicMock,
+) -> None:
+    """A persistent network error is retried, then degrades to broadening-off:
+    only the seed comes back, and the retries were actually attempted."""
+    # Arrange
+    client = PostcodesIoClient()
+    coords = Coordinates(longitude=0.1, latitude=51.3)
+
+    with patch(
+        f"{_MODULE}.httpx.get", side_effect=httpx.ConnectError("down")
+    ) as mock_get:
+        # Act
+        result = client.nearby("BR6 6BS", coords)
+
+    # Assert — one initial try + MAX_RETRIES, sleeping between each.
+    assert result == ["BR6 6BS"]
+    assert mock_get.call_count == client.MAX_RETRIES + 1
+    assert _no_sleep.call_count == client.MAX_RETRIES
+
+
+def test_nearby_retries_a_transport_error_then_succeeds(_no_sleep: MagicMock) -> None:
+    """A transient transport error is retried, and the subsequent success is
+    returned in full."""
+    # Arrange
+    client = PostcodesIoClient()
+    coords = Coordinates(longitude=0.1, latitude=51.3)
+
+    with patch(f"{_MODULE}.httpx.get") as mock_get:
+        mock_get.side_effect = [
+            httpx.ReadTimeout("slow"),
+            _response(_nearest_payload(["BR6 6BS", "BR6 6BU"])),
+        ]
+
+        # Act
+        result = client.nearby("BR6 6BS", coords)
+
+    # Assert
+    assert result == ["BR6 6BS", "BR6 6BU"]
+    assert mock_get.call_count == 2
+    assert _no_sleep.call_count == 1
+
+
+def test_nearby_retries_a_429_honouring_retry_after(_no_sleep: MagicMock) -> None:
+    """A 429 is retried, and the server's Retry-After drives the backoff delay."""
+    # Arrange
+    client = PostcodesIoClient()
+    coords = Coordinates(longitude=0.1, latitude=51.3)
+
+    with patch(f"{_MODULE}.httpx.get") as mock_get:
+        mock_get.side_effect = [
+            _response(None, status_code=429, headers={"Retry-After": "2"}),
+            _response(_nearest_payload(["BR6 6BS", "BR6 6BU"])),
+        ]
+
+        # Act
+        result = client.nearby("BR6 6BS", coords)
+
+    # Assert — succeeded on the retry, having slept the advertised 2 seconds.
+    assert result == ["BR6 6BS", "BR6 6BU"]
+    assert mock_get.call_count == 2
+    _no_sleep.assert_called_once_with(2.0)
+
+
+def test_nearby_retries_a_server_error_then_succeeds(_no_sleep: MagicMock) -> None:
+    """A 5xx is treated as transient and retried."""
+    # Arrange
+    client = PostcodesIoClient()
+    coords = Coordinates(longitude=0.1, latitude=51.3)
+
+    with patch(f"{_MODULE}.httpx.get") as mock_get:
+        mock_get.side_effect = [
+            _response(None, status_code=503),
+            _response(_nearest_payload(["BR6 6BS"])),
+        ]
+
+        # Act
+        result = client.nearby("BR6 6BS", coords)
+
+    # Assert
+    assert result == ["BR6 6BS"]
+    assert mock_get.call_count == 2
+
+
+def test_nearby_returns_just_the_seed_when_centroid_unresolvable() -> None:
+    """An unknown seed (no coordinates, centroid lookup fails) yields the seed
+    alone rather than raising."""
+    # Arrange
+    client = PostcodesIoClient()
+
+    with patch(f"{_MODULE}.httpx.get") as mock_get:
+        mock_get.return_value = _response(None, status_code=404)
+
+        # Act
+        result: list[str] = client.nearby("ZZ99 9ZZ")
+
+    # Assert — a 404 is non-transient, so no retry was attempted.
+    assert result == ["ZZ99 9ZZ"]
+    assert mock_get.call_count == 1
+
+
+def test_nearby_tolerates_a_null_nearest_result() -> None:
+    """postcodes.io returns ``result: null`` when a point has no neighbours; the
+    client treats that as an empty neighbour set (seed only)."""
+    # Arrange
+    client = PostcodesIoClient()
+    coords: Optional[Coordinates] = Coordinates(longitude=0.1, latitude=51.3)
+
+    with patch(f"{_MODULE}.httpx.get") as mock_get:
+        mock_get.return_value = _response({"result": None})
+
+        # Act
+        result = client.nearby("BR6 6BS", coords)
+
+    # Assert
+    assert result == ["BR6 6BS"]
diff --git a/tests/infrastructure/test_http_retry.py b/tests/infrastructure/test_http_retry.py
new file mode 100644
index 00000000..ec1e229f
--- /dev/null
+++ b/tests/infrastructure/test_http_retry.py
@@ -0,0 +1,126 @@
+"""call_with_retry — the shared transient-failure retry both HTTP source clients
+use. Generic mechanism (exponential backoff, Retry-After), per-client policy
+(what gets raised as transient)."""
+
+from __future__ import annotations
+
+from typing import Iterator
+from unittest.mock import MagicMock, call, patch
+
+import httpx
+import pytest
+
+from infrastructure.http_retry import TransientHttpError, call_with_retry
+
+
+@pytest.fixture(autouse=True)
+def _no_sleep() -> Iterator[MagicMock]:
+    with patch("infrastructure.http_retry.time.sleep") as sleep:
+        yield sleep
+
+
+def test_returns_immediately_on_success(_no_sleep: MagicMock) -> None:
+    # Act
+    result = call_with_retry(lambda: 42)
+
+    # Assert
+    assert result == 42
+    _no_sleep.assert_not_called()
+
+
+def test_retries_a_transient_error_then_succeeds(_no_sleep: MagicMock) -> None:
+    # Arrange — fail twice transiently, then succeed.
+    calls = {"n": 0}
+
+    def fn() -> str:
+        calls["n"] += 1
+        if calls["n"] < 3:
+            raise TransientHttpError("429")
+        return "ok"
+
+    # Act
+    result = call_with_retry(fn)
+
+    # Assert — exponential backoff between the three attempts.
+    assert result == "ok"
+    assert _no_sleep.call_args_list == [call(1.0), call(2.0)]
+
+
+def test_honours_retry_after_over_the_computed_backoff(_no_sleep: MagicMock) -> None:
+    # Arrange
+    raised = {"done": False}
+
+    def fn() -> str:
+        if not raised["done"]:
+            raised["done"] = True
+            raise TransientHttpError("429", retry_after=7.0)
+        return "ok"
+
+    # Act
+    call_with_retry(fn)
+
+    # Assert
+    _no_sleep.assert_called_once_with(7.0)
+
+
+def test_retries_transport_errors(_no_sleep: MagicMock) -> None:
+    # Arrange
+    attempts = {"n": 0}
+
+    def fn() -> str:
+        attempts["n"] += 1
+        if attempts["n"] == 1:
+            raise httpx.ReadTimeout("slow")
+        return "ok"
+
+    # Act
+    result = call_with_retry(fn)
+
+    # Assert
+    assert result == "ok"
+    assert attempts["n"] == 2
+
+
+def test_reraises_the_last_transient_error_once_exhausted(
+    _no_sleep: MagicMock,
+) -> None:
+    # Arrange — always transient.
+    def fn() -> str:
+        raise TransientHttpError("persistent 429")
+
+    # Act / Assert
+    with pytest.raises(TransientHttpError, match="persistent 429"):
+        call_with_retry(fn, max_retries=2)
+    assert _no_sleep.call_count == 2
+
+
+def test_non_transient_error_propagates_without_retry(_no_sleep: MagicMock) -> None:
+    # Arrange
+    attempts = {"n": 0}
+
+    def fn() -> str:
+        attempts["n"] += 1
+        raise ValueError("not transient")
+
+    # Act / Assert
+    with pytest.raises(ValueError, match="not transient"):
+        call_with_retry(fn)
+    assert attempts["n"] == 1
+    _no_sleep.assert_not_called()
+
+
+def test_retry_after_is_capped_by_max_backoff(_no_sleep: MagicMock) -> None:
+    # Arrange
+    raised = {"done": False}
+
+    def fn() -> str:
+        if not raised["done"]:
+            raised["done"] = True
+            raise TransientHttpError("429", retry_after=999.0)
+        return "ok"
+
+    # Act
+    call_with_retry(fn, max_backoff=60.0)
+
+    # Assert
+    _no_sleep.assert_called_once_with(60.0)
diff --git a/tests/repositories/comparable_properties/test_epc_comparable_properties_repository.py b/tests/repositories/comparable_properties/test_epc_comparable_properties_repository.py
index 418b2af6..1a0b918e 100644
--- a/tests/repositories/comparable_properties/test_epc_comparable_properties_repository.py
+++ b/tests/repositories/comparable_properties/test_epc_comparable_properties_repository.py
@@ -130,6 +130,126 @@ def test_no_certs_in_the_postcode_yields_no_candidates() -> None:
     assert client.searched_postcode == "LS6 1AA"
 
 
+# ---------------------------------------------------------------------------
+# Broadened cohort — candidates_near (ADR-0034 nearby-postcode broadening)
+# ---------------------------------------------------------------------------
+
+
+class _MultiPostcodeEpcClient:
+    """Serves a different cohort per postcode and records every search, so the
+    broadened walk's reach and ordering can be asserted."""
+
+    def __init__(self, by_postcode: dict[str, list[EpcSearchResult]]) -> None:
+        self._by_postcode = by_postcode
+        self.searched: list[str] = []
+
+    def search_by_postcode(self, postcode: str) -> list[EpcSearchResult]:
+        self.searched.append(postcode)
+        return self._by_postcode.get(postcode, [])
+
+    def get_by_certificate_number(self, cert_num: str) -> EpcPropertyData:
+        return _epc()
+
+
+class _FakeNearbyPostcodes:
+    """Returns a fixed nearest-first list and records the seed it was asked for."""
+
+    def __init__(self, postcodes: list[str]) -> None:
+        self._postcodes = postcodes
+        self.calls: list[tuple[str, Optional[Coordinates]]] = []
+
+    def nearby(
+        self, postcode: str, coordinates: Optional[Coordinates] = None
+    ) -> list[str]:
+        self.calls.append((postcode, coordinates))
+        return self._postcodes
+
+
+def test_candidates_near_aggregates_and_dedupes_across_nearby_postcodes() -> None:
+    # Arrange — three nearby postcodes; CERT-1 is re-lodged in two of them.
+    client = _MultiPostcodeEpcClient(
+        {
+            "P0": [_result("CERT-1", uprn=1)],
+            "P1": [_result("CERT-2", uprn=2), _result("CERT-1", uprn=1)],
+            "P2": [_result("CERT-3", uprn=3)],
+        }
+    )
+    nearby = _FakeNearbyPostcodes(["P0", "P1", "P2"])
+    repo = EpcComparablePropertiesRepository(
+        client, _FakeGeospatial({}), nearby_postcodes=nearby
+    )
+
+    # Act — no early-stop predicate, so the whole nearby set is visited.
+    candidates = repo.candidates_near("P0", None)
+
+    # Assert — one candidate per distinct cert, all three postcodes searched.
+    certs = {c.certificate_number for c in candidates}
+    assert certs == {"CERT-1", "CERT-2", "CERT-3"}
+    assert client.searched == ["P0", "P1", "P2"]
+
+
+def test_candidates_near_stops_early_once_enough_match() -> None:
+    # Arrange — the seed postcode alone already yields enough matches; the two
+    # further postcodes must not be fetched.
+    client = _MultiPostcodeEpcClient(
+        {
+            "P0": [_result(f"MATCH-{i}", uprn=i) for i in range(5)],
+            "P1": [_result("OTHER-1", uprn=99)],
+            "P2": [_result("OTHER-2", uprn=98)],
+        }
+    )
+    nearby = _FakeNearbyPostcodes(["P0", "P1", "P2"])
+    repo = EpcComparablePropertiesRepository(
+        client, _FakeGeospatial({}), nearby_postcodes=nearby
+    )
+
+    # Act
+    candidates = repo.candidates_near(
+        "P0",
+        None,
+        enough=lambda c: c.certificate_number.startswith("MATCH"),
+        minimum=5,
+    )
+
+    # Assert — walk halted after the seed; the further postcodes were never hit.
+    assert client.searched == ["P0"]
+    assert len(candidates) == 5
+
+
+def test_candidates_near_passes_coordinates_to_the_nearby_source() -> None:
+    # Arrange
+    here = Coordinates(longitude=0.1, latitude=51.3)
+    client = _MultiPostcodeEpcClient({"P0": []})
+    nearby = _FakeNearbyPostcodes(["P0"])
+    repo = EpcComparablePropertiesRepository(
+        client, _FakeGeospatial({}), nearby_postcodes=nearby
+    )
+
+    # Act
+    repo.candidates_near("P0", here)
+
+    # Assert — the target's own coordinates seed the radius search.
+    assert nearby.calls == [("P0", here)]
+
+
+def test_candidates_near_without_a_source_uses_only_the_seed() -> None:
+    # Arrange — no NearbyPostcodes configured (broadening unavailable).
+    client = _MultiPostcodeEpcClient({"P0": [_result("CERT-1", uprn=1)]})
+    repo = EpcComparablePropertiesRepository(client, _FakeGeospatial({}))
+
+    # Act
+    candidates = repo.candidates_near("P0", None)
+
+    # Assert — degrades to the seed postcode alone.
+    assert client.searched == ["P0"]
+    assert [c.certificate_number for c in candidates] == ["CERT-1"]
+
+
+# ---------------------------------------------------------------------------
+# Unmappable cohort certs are skipped + recorded, not allowed to sink the cohort
+# ---------------------------------------------------------------------------
+
+
 class _FakeEpcClientRaising:
     """Serves the cohort, but raises the given exception for specific certs."""