SAP calculator entry point + cert→inputs adapter + strict P6.1 identifiers

Lands the production code that the just-committed Elmhurst conformance
fixtures (6455d48b) exercise: the SAP10.3 calculator orchestrator
(domain.sap.calculator.Sap10Calculator), the RdSAP-driven cert→inputs
mapper (domain.sap.rdsap.cert_to_inputs), and the EpcPropertyData
strict-type pass that P6.1 starts.

calculator.py is the entry point. Two surfaces depending on the caller's
shape:
- Sap10Calculator().calculate(epc) — full RdSAP mapper + worksheet loop
- calculate_sap_from_inputs(inputs) — pure physics over typed inputs

P6.1 introduces BuildingPartIdentifier as a strictly-typed replacement
for bare-string matching on SapBuildingPart.identifier (motivated by
the pain point at worksheet/dimensions.py:74-82). Two boundary factories
canonicalise raw inputs: from_api_string for the gov-EPC API, and
extension(n) for site-notes / construction id flows.

Also catches up two transitive deps that 6455d48b implicitly required
but I missed:
- ml/rdsap_uvalues.py — party-wall U-value rows that heat_transmission
  resolves; the U=0.0 branch the 000516 fixture exercises lands here.
- ml/tests/_fixtures.py — make_minimal_sap10_epc that every Elmhurst
  fixture imports. Without this catch-up, checking out 6455d48b in
  isolation would ImportError.

Out of scope (will commit separately): ml/transform.py legacy envelope
drift; backend/ FastAPI + documents_parser layer; etl/ scratch.

824 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

datatypes/epc/domain/epc_property_data.py

@@ -1,10 +1,67 @@
+import re
 from dataclasses import dataclass
 from datetime import date
-from typing import List, Optional, Union
+from enum import Enum
+from typing import Final, List, Optional, Union
 
 from datatypes.epc.domain.epc import Epc
 
 
+_API_EXTENSION = re.compile(r"^Extension\s+(\d+)$")
+
+
+class BuildingPartIdentifier(Enum):
+    """Canonical identifier for a SAP building part.
+
+    Replaces bare-string matching on `SapBuildingPart.identifier`. The
+    enum *values* match the site-notes / database shape ("main",
+    "extension_1" .. "extension_4"); boundary mappers (gov-EPC API,
+    site notes) construct these via the `from_api_string` / `extension`
+    classmethods so consumers can dispatch with `is` instead of fragile
+    string equality.
+
+    RdSAP10 §1.2 caps extensions at 4 per dwelling, so EXTENSION_1..4
+    are enumerated explicitly; anything else falls to OTHER so callers
+    can still iterate safely.
+
+    P6.1 — first slice of the strict-typing P6 work documented in
+    HANDOVER_SYSTEMATIC_REVIEW §2.5.
+    """
+
+    MAIN = "main"
+    EXTENSION_1 = "extension_1"
+    EXTENSION_2 = "extension_2"
+    EXTENSION_3 = "extension_3"
+    EXTENSION_4 = "extension_4"
+    OTHER = "other"
+
+    @classmethod
+    def from_api_string(
+        cls, api_identifier: Optional[str]
+    ) -> "BuildingPartIdentifier":
+        """Map a gov-EPC API `BuildingPart.identifier` to its canonical
+        member. "Main Dwelling" → MAIN; "Extension N" → EXTENSION_N
+        (for N in 1..4). `None` (permitted by the 21_0_1 schema) and
+        anything unrecognised fall to OTHER.
+        """
+        if api_identifier == "Main Dwelling":
+            return cls.MAIN
+        if api_identifier is not None:
+            match = _API_EXTENSION.match(api_identifier)
+            if match is not None:
+                return cls.extension(int(match.group(1)))
+        return cls.OTHER
+
+    @classmethod
+    def extension(cls, n: int) -> "BuildingPartIdentifier":
+        """Canonical identifier for the Nth extension. RdSAP10 §1.2
+        caps at 4; numbers outside 1..4 fall to OTHER."""
+        try:
+            return cls(f"extension_{n}")
+        except ValueError:
+            return cls.OTHER
+
+
 @dataclass
 class EnergyElement:
     description: str
@@ -201,6 +258,14 @@ class SapRoomInRoof:
     construction_age_band: str
 
 
+# RdSAP10 wall_construction integer encoding. The gov-EPC API doesn't publish
+# the mapping; established empirically from a 50k 2026-bulk sweep — code 6
+# co-occurs with `walls[].description = "Basement wall"` in 88% of cases at
+# a 0.18% false-positive rate, so we treat it as the canonical basement-wall
+# signal.
+BASEMENT_WALL_CONSTRUCTION_CODE: Final[int] = 6
+
+
 @dataclass
 class SapAlternativeWall:
     wall_area: float
@@ -210,11 +275,19 @@ class SapAlternativeWall:
     wall_thickness_measured: str
     wall_insulation_thickness: Optional[str] = None
 
+    @property
+    def is_basement_wall(self) -> bool:
+        """True iff this alt sub-area is the dwelling's basement wall —
+        identified by RdSAP10 wall_construction code = 6 (see module
+        constant `BASEMENT_WALL_CONSTRUCTION_CODE`). RdSAP §5.17 / Table 23
+        applies a special U-value lookup to basement walls."""
+        return self.wall_construction == BASEMENT_WALL_CONSTRUCTION_CODE
+
 
 @dataclass
 class SapBuildingPart:
     # General
-    identifier: str  # e.g. "main", "roof"
+    identifier: BuildingPartIdentifier
     construction_age_band: str
 
     # Wall
@@ -261,6 +334,29 @@ class SapBuildingPart:
     )
     sap_room_in_roof: Optional[SapRoomInRoof] = None
 
+    @property
+    def main_wall_is_basement(self) -> bool:
+        """True iff this part's primary wall (not an alt sub-area) is the
+        basement wall — happens when the whole part sits below grade.
+        Empirically 54 of 67k parts in the 2026 sweep; rare but real."""
+        return self.wall_construction == BASEMENT_WALL_CONSTRUCTION_CODE
+
+    @property
+    def has_basement(self) -> bool:
+        """True iff this part carries a basement wall — either as its
+        main wall (`main_wall_is_basement`) or as an alt sub-area
+        (`SapAlternativeWall.is_basement_wall`). When true, RdSAP §5.17 /
+        Table 23 governs both the basement-wall U-value AND the entire
+        ground floor's U-value for this part (per user-confirmed
+        convention: basement-wall presence ⇒ whole floor=0 is basement
+        floor)."""
+        if self.main_wall_is_basement:
+            return True
+        return any(
+            alt is not None and alt.is_basement_wall
+            for alt in (self.sap_alternative_wall_1, self.sap_alternative_wall_2)
+        )
+
 
 @dataclass
 class WindowsTransmissionDetails:

datatypes/epc/domain/mapper.py

@@ -4,6 +4,7 @@ from datatypes.epc.schema.helpers import from_dict
 
 from datatypes.epc.domain.epc_property_data import (
     Addendum,
+    BuildingPartIdentifier,
     EnergyElement,
     EpcPropertyData,
     InstantaneousWwhrs,
@@ -427,7 +428,7 @@ class EpcPropertyDataMapper:
             ),
             sap_building_parts=[
                 SapBuildingPart(
-                    identifier=bp.identifier,
+                    identifier=BuildingPartIdentifier.from_api_string(bp.identifier),
                     construction_age_band=bp.construction_age_band,
                     wall_construction=bp.wall_construction,
                     wall_insulation_type=bp.wall_insulation_type,
@@ -568,7 +569,7 @@ class EpcPropertyDataMapper:
             ),
             sap_building_parts=[
                 SapBuildingPart(
-                    identifier=bp.identifier,
+                    identifier=BuildingPartIdentifier.from_api_string(bp.identifier),
                     construction_age_band=bp.construction_age_band,
                     wall_construction=bp.wall_construction,
                     wall_insulation_type=bp.wall_insulation_type,
@@ -705,7 +706,7 @@ class EpcPropertyDataMapper:
             ),
             sap_building_parts=[
                 SapBuildingPart(
-                    identifier=bp.identifier,
+                    identifier=BuildingPartIdentifier.from_api_string(bp.identifier),
                     construction_age_band=bp.construction_age_band,
                     wall_construction=bp.wall_construction,
                     wall_insulation_type=bp.wall_insulation_type,
@@ -850,7 +851,7 @@ class EpcPropertyDataMapper:
             ),
             sap_building_parts=[
                 SapBuildingPart(
-                    identifier=bp.identifier,
+                    identifier=BuildingPartIdentifier.from_api_string(bp.identifier),
                     construction_age_band=bp.construction_age_band,
                     wall_construction=bp.wall_construction,
                     wall_insulation_type=bp.wall_insulation_type,
@@ -1012,7 +1013,7 @@ class EpcPropertyDataMapper:
             ),
             sap_building_parts=[
                 SapBuildingPart(
-                    identifier=bp.identifier,
+                    identifier=BuildingPartIdentifier.from_api_string(bp.identifier),
                     construction_age_band=bp.construction_age_band,
                     wall_construction=bp.wall_construction,
                     wall_insulation_type=bp.wall_insulation_type,
@@ -1201,7 +1202,7 @@ class EpcPropertyDataMapper:
             ),
             sap_building_parts=[
                 SapBuildingPart(
-                    identifier=bp.identifier,
+                    identifier=BuildingPartIdentifier.from_api_string(bp.identifier),
                     construction_age_band=bp.construction_age_band,
                     wall_construction=bp.wall_construction,
                     wall_insulation_type=bp.wall_insulation_type,
@@ -1446,7 +1447,7 @@ class EpcPropertyDataMapper:
             # SAP building parts
             sap_building_parts=[
                 SapBuildingPart(
-                    identifier=bp.identifier,
+                    identifier=BuildingPartIdentifier.from_api_string(bp.identifier),
                     construction_age_band=bp.construction_age_band,
                     wall_construction=bp.wall_construction,
                     wall_insulation_type=bp.wall_insulation_type,
@@ -1755,7 +1756,7 @@ def _map_main_building_part(
     floor = construction.floor
     roof_location, roof_thickness = _map_roof(roof)
     return SapBuildingPart(
-        identifier="main",
+        identifier=BuildingPartIdentifier.MAIN,
         construction_age_band=_extract_age_band(main.age_range),
         wall_construction=main.walls_construction_type,
         wall_insulation_type=main.walls_insulation_type,
@@ -1779,7 +1780,7 @@ def _map_extension_building_part(
 ) -> SapBuildingPart:
     roof_location, roof_thickness = _map_roof(roof)
     return SapBuildingPart(
-        identifier=f"extension_{ext_c.id}",
+        identifier=BuildingPartIdentifier.extension(ext_c.id),
         construction_age_band=_extract_age_band(ext_c.age_range),
         wall_construction=ext_c.walls_construction_type,
         wall_insulation_type=ext_c.walls_insulation_type,
@@ -1902,7 +1903,7 @@ def _map_elmhurst_building_part(survey: ElmhurstSiteNotes) -> SapBuildingPart:
         for i, f in enumerate(dims.floors)
     ]
     return SapBuildingPart(
-        identifier="main",
+        identifier=BuildingPartIdentifier.MAIN,
         construction_age_band=_strip_code(survey.construction_age_band),
         wall_construction=_strip_code(survey.walls.wall_type),
         wall_insulation_type=_strip_code(survey.walls.insulation),

datatypes/epc/domain/tests/test_building_part_identifier.py

@@ -0,0 +1,98 @@
+"""Tests for `BuildingPartIdentifier` — the strictly-typed identifier
+that replaces bare-string matching on `SapBuildingPart.identifier`.
+
+Two boundary factories convert raw inputs to canonical members:
+- `BuildingPartIdentifier.from_api_string` (gov-EPC API)
+- `BuildingPartIdentifier.extension(n)` (site-notes / construction id)
+
+P6.1 starts P6 (strict-type EpcPropertyData) from the documented pain
+point in packages/domain/src/domain/sap/worksheet/dimensions.py:74-82.
+"""
+from __future__ import annotations
+
+import pytest
+
+from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+
+
+class TestFromApiString:
+    """The gov-EPC API returns "Main Dwelling" and "Extension N"; the
+    21_0_1 schema also permits `None`. All map to canonical members."""
+
+    def test_main_dwelling_becomes_main(self) -> None:
+        # Arrange / Act
+        identifier = BuildingPartIdentifier.from_api_string("Main Dwelling")
+
+        # Assert
+        assert identifier is BuildingPartIdentifier.MAIN
+
+    @pytest.mark.parametrize(
+        "api_string, expected",
+        [
+            ("Extension 1", BuildingPartIdentifier.EXTENSION_1),
+            ("Extension 2", BuildingPartIdentifier.EXTENSION_2),
+            ("Extension 3", BuildingPartIdentifier.EXTENSION_3),
+            ("Extension 4", BuildingPartIdentifier.EXTENSION_4),
+        ],
+    )
+    def test_extension_n_becomes_extension_n(
+        self, api_string: str, expected: BuildingPartIdentifier
+    ) -> None:
+        # Arrange / Act
+        identifier = BuildingPartIdentifier.from_api_string(api_string)
+
+        # Assert
+        assert identifier is expected
+
+    def test_none_becomes_other(self) -> None:
+        # Arrange — the 21_0_1 schema permits `identifier: Optional[str]`.
+        # Act
+        identifier = BuildingPartIdentifier.from_api_string(None)
+
+        # Assert
+        assert identifier is BuildingPartIdentifier.OTHER
+
+    @pytest.mark.parametrize(
+        "api_string", ["", "roof", "garage", "Extension", "Main", "Extension 5"]
+    )
+    def test_unrecognised_becomes_other(self, api_string: str) -> None:
+        # Arrange — "Extension 5" is intentionally OTHER per RdSAP10 §1.2
+        # (max 4 extensions); bare "Extension" with no digit likewise.
+        # Act
+        identifier = BuildingPartIdentifier.from_api_string(api_string)
+
+        # Assert
+        assert identifier is BuildingPartIdentifier.OTHER
+
+
+class TestExtensionFactory:
+    """`extension(n)` is the site-notes-side constructor — surveyors
+    record extensions by integer id; this maps id→canonical member."""
+
+    @pytest.mark.parametrize(
+        "n, expected",
+        [
+            (1, BuildingPartIdentifier.EXTENSION_1),
+            (2, BuildingPartIdentifier.EXTENSION_2),
+            (3, BuildingPartIdentifier.EXTENSION_3),
+            (4, BuildingPartIdentifier.EXTENSION_4),
+        ],
+    )
+    def test_valid_extension_number_returns_member(
+        self, n: int, expected: BuildingPartIdentifier
+    ) -> None:
+        # Arrange / Act
+        identifier = BuildingPartIdentifier.extension(n)
+
+        # Assert
+        assert identifier is expected
+
+    @pytest.mark.parametrize("n", [0, 5, 99, -1])
+    def test_out_of_range_falls_to_other(self, n: int) -> None:
+        # Arrange — RdSAP10 §1.2 caps at 4; out-of-range numbers should
+        # not crash the mapper, they should classify as OTHER.
+        # Act
+        identifier = BuildingPartIdentifier.extension(n)
+
+        # Assert
+        assert identifier is BuildingPartIdentifier.OTHER

datatypes/epc/domain/tests/test_from_site_notes.py

@@ -6,6 +6,7 @@ from typing import Any, Dict
 import pytest
 
 from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
     EpcPropertyData,
     InstantaneousWwhrs,
     MainHeatingDetail,
@@ -211,7 +212,7 @@ class TestFromSiteNotesExample1:
         assert len(result.sap_building_parts) == 1
 
     def test_building_part_identifier(self, result: EpcPropertyData) -> None:
-        assert result.sap_building_parts[0].identifier == "main"
+        assert result.sap_building_parts[0].identifier is BuildingPartIdentifier.MAIN
 
     def test_construction_age_band(self, result: EpcPropertyData) -> None:
         # main_building.age_range: "I: 1996 - 2002" → letter "I"
@@ -464,7 +465,7 @@ class TestFromSiteNotesExample1:
             # Building parts
             sap_building_parts=[
                 SapBuildingPart(
-                    identifier="main",
+                    identifier=BuildingPartIdentifier.MAIN,
                     construction_age_band="I",
                     wall_construction="Cavity",
                     wall_insulation_type="As built",

packages/domain/src/domain/ml/rdsap_uvalues.py

@@ -587,6 +587,59 @@ def u_door(
 # ---------------------------------------------------------------------------
 
 
+# ---------------------------------------------------------------------------
+# Basement U-values  (RdSAP §5.17 / Table 23)
+#
+# Applied when a building part carries an alt wall with
+# wall_construction == BASEMENT_WALL_CONSTRUCTION_CODE. Per the user-
+# confirmed convention, the WHOLE floor=0 of that part is the basement
+# floor — so `u_basement_floor` overrides the regular floor U-value for
+# the part's ground floor area, and `u_basement_wall` overrides the
+# cascade for the basement alt sub-area only.
+# ---------------------------------------------------------------------------
+
+
+_BASEMENT_WALL_BY_BAND: Final[dict[str, float]] = {
+    "A": 0.7, "B": 0.7, "C": 0.7, "D": 0.7, "E": 0.7,
+    "F": 0.7,
+    "G": 0.6, "H": 0.6,
+    "I": 0.45,
+    "J": 0.35,
+    "K": 0.3,
+    "L": 0.28,
+    "M": 0.26,
+}
+
+_BASEMENT_FLOOR_BY_BAND: Final[dict[str, float]] = {
+    "A": 0.50, "B": 0.50, "C": 0.50, "D": 0.50, "E": 0.50,
+    "F": 0.50,
+    "G": 0.5, "H": 0.5, "I": 0.5,
+    "J": 0.25,
+    "K": 0.22,
+    "L": 0.22,
+    "M": 0.18,
+}
+
+
+def u_basement_wall(age_band: Optional[str]) -> float:
+    """Basement-wall U-value (W/m²K), RdSAP10 Table 23. Defaults to the
+    A-E value (0.7) when age band is missing — matches the worst-case
+    cascade behaviour elsewhere in this module."""
+    if age_band is None:
+        return 0.7
+    return _BASEMENT_WALL_BY_BAND.get(age_band.upper(), 0.7)
+
+
+def u_basement_floor(age_band: Optional[str]) -> float:
+    """Basement-floor U-value (W/m²K), RdSAP10 Table 23. Applied to the
+    WHOLE floor=0 of a part that has a basement (per user-confirmed
+    convention: basement-wall presence ⇒ entire ground floor is basement
+    floor). Defaults to the A-E value (0.50) when band is missing."""
+    if age_band is None:
+        return 0.50
+    return _BASEMENT_FLOOR_BY_BAND.get(age_band.upper(), 0.50)
+
+
 def u_party_wall(party_wall_construction: Optional[int]) -> float:
     """RdSAP10 party-wall U-value in W/m^2K, never null.
 

packages/domain/src/domain/ml/tests/_fixtures.py

@@ -12,6 +12,7 @@ from datetime import date
 from typing import Optional, Union
 
 from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
     EpcPropertyData,
     InstantaneousWwhrs,
     MainHeatingDetail,
@@ -123,7 +124,7 @@ def make_floor_dimension(
 
 def make_building_part(
     *,
-    identifier: str = "Main Dwelling",
+    identifier: BuildingPartIdentifier = BuildingPartIdentifier.MAIN,
     construction_age_band: str = "B",
     wall_construction: Union[int, str] = 3,
     wall_insulation_type: Union[int, str] = 2,

packages/domain/src/domain/sap/calculator.py

@@ -96,7 +96,11 @@ class CalculatorInputs:
 
     dimensions: Dimensions
     heat_transmission: HeatTransmission
-    infiltration_ach: float
+    # SAP10.2 (25)m — effective monthly air-change rate (12-tuple Jan..Dec).
+    # Per-month because ventilation HLC varies with wind speed (Table U2)
+    # and MV mode (§2 lines 24a-d). Constant-monthly inputs work too:
+    # pass `(ach,) * 12` to model a single rate across all months.
+    monthly_infiltration_ach: tuple[float, ...]
     region: int
     windows: tuple[WindowInput, ...]
     control_type: int
@@ -278,22 +282,34 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
     typed `SapResult`. Cert-shape mapping is the job of `cert_to_inputs`
     (S-A7b); this entry point is pure physics."""
     tfa = inputs.dimensions.total_floor_area_m2
-    hlc_v = inputs.infiltration_ach * inputs.dimensions.volume_m3 * _AIR_HEAT_CAPACITY_WH_PER_M3_K
+    volume = inputs.dimensions.volume_m3
-    hlc = inputs.heat_transmission.total_w_per_k + hlc_v
+    transmission_hlc = inputs.heat_transmission.total_w_per_k
-    hlp = hlc / tfa if tfa > 0 else 0.0
+
-    tau_h = _time_constant_h(
+    # SAP10.2 §3 line (38): ventilation HLC = 0.33 × (25)m × volume —
-        tmp_kj_per_m2_k=inputs.thermal_mass_parameter_kj_per_m2_k,
+    # monthly because (25)m varies with Table U2 wind. HLC, HLP, and the
-        tfa_m2=tfa,
+    # time constant τ all become 12-tuples.
-        hlc_w_per_k=hlc,
+    monthly_hlc_v = tuple(
+        ach * volume * _AIR_HEAT_CAPACITY_WH_PER_M3_K
+        for ach in inputs.monthly_infiltration_ach
+    )
+    monthly_hlc = tuple(transmission_hlc + hv for hv in monthly_hlc_v)
+    monthly_hlp = tuple(h / tfa if tfa > 0 else 0.0 for h in monthly_hlc)
+    monthly_tau_h = tuple(
+        _time_constant_h(
+            tmp_kj_per_m2_k=inputs.thermal_mass_parameter_kj_per_m2_k,
+            tfa_m2=tfa,
+            hlc_w_per_k=h,
+        )
+        for h in monthly_hlc
     )
 
     monthly = tuple(
         _solve_month(
             inputs=inputs,
             month=m,
-            hlc_w_per_k=hlc,
+            hlc_w_per_k=monthly_hlc[m - 1],
-            time_constant_h=tau_h,
+            time_constant_h=monthly_tau_h[m - 1],
-            heat_loss_parameter=hlp,
+            heat_loss_parameter=monthly_hlp[m - 1],
         )
         for m in range(1, 13)
     )
@@ -374,11 +390,13 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "windows_w_per_k": ht.windows_w_per_k,
         "doors_w_per_k": ht.doors_w_per_k,
         "thermal_bridging_w_per_k": ht.thermal_bridging_w_per_k,
-        "infiltration_ach": inputs.infiltration_ach,
+        # Annual means for the back-compat single-float audit dict; full
-        "infiltration_w_per_k": hlc_v,
+        # monthly arrays are available via the upstream VentilationResult.
-        "heat_transfer_coefficient_w_per_k": hlc,
+        "infiltration_ach": sum(inputs.monthly_infiltration_ach) / 12.0,
-        "heat_loss_parameter_w_per_m2k": hlp,
+        "infiltration_w_per_k": sum(monthly_hlc_v) / 12.0,
-        "time_constant_h": tau_h,
+        "heat_transfer_coefficient_w_per_k": sum(monthly_hlc) / 12.0,
+        "heat_loss_parameter_w_per_m2k": sum(monthly_hlp) / 12.0,
+        "time_constant_h": sum(monthly_tau_h) / 12.0,
         "internal_gains_annual_avg_w": sum(e.internal_gains_w for e in monthly) / 12.0,
         "mean_internal_temp_annual_avg_c": sum(e.internal_temp_c for e in monthly) / 12.0,
         "useful_space_heating_kwh_per_yr": space_heating_kwh,

packages/domain/src/domain/sap/rdsap/cert_to_inputs.py

@@ -64,7 +64,10 @@ from domain.sap.worksheet.heat_transmission import (
     heat_transmission_from_cert,
 )
 from domain.sap.worksheet.solar_gains import Orientation
-from domain.sap.worksheet.ventilation import infiltration_ach
+from domain.sap.worksheet.ventilation import (
+    MechanicalVentilationKind,
+    ventilation_from_inputs,
+)
 
 
 # RdSAP 10 Table 27 — fraction of total floor area treated as the
@@ -766,7 +769,35 @@ def cert_to_inputs(
     vol = dim.volume_m3 if dim.volume_m3 > 0 else 1.0
     storeys = max(1, dim.storey_count)
     vc = _ventilation_counts(epc.sap_ventilation)
-    infiltration = infiltration_ach(
+    # SAP §2 ventilation: full worksheet (6a)..(25)m via VentilationResult.
+    # The following cert→input ambiguities are intentionally papered over
+    # with spec-default values for now; each TODO is a tracked follow-up
+    # for when the mapping rule becomes available:
+    #
+    # TODO(cert→ventilation 1): `epc.mechanical_ventilation: int` carries
+    #   a code (0..N) selecting Natural / MVHR / MV / Extract / PIV-outside
+    #   / PIV-loft. The int→enum mapping isn't in the SAP10.2 or RdSAP10
+    #   PDFs we have; Elmhurst likely uses the same code list. We pin
+    #   NATURAL until we have a documented mapping or a golden cert that
+    #   exercises an MV path.
+    # TODO(cert→ventilation 2): `has_suspended_timber_floor` / `_sealed`
+    #   should be derived from `floor_construction` + `floor_insulation`
+    #   on `SapFloorDimension`; the RdSAP §4.1 rule for "treat as
+    #   suspended timber" isn't yet wired in. Defaulted to False.
+    # TODO(cert→ventilation 3): `has_draught_lobby` is not lodged on the
+    #   gov-EPC API. RdSAP §4.1 may infer from dwelling form. Defaulted
+    #   to False (worst case for line (13) = 0.05).
+    # TODO(cert→ventilation 4): `air_permeability_ap50` / `ap4` from a
+    #   pressure test — cert has `pressure_test: int` (code, not a value)
+    #   and `air_tightness: {description,...}`. Likely only present on
+    #   SAP (new-build) certs, not RdSAP. Defaulted to None (no test).
+    # TODO(cert→ventilation 5): `sheltered_sides` not on the cert — we
+    #   hardcode 2 (typical UK terraced/semi-detached). Could be derived
+    #   from `dwelling_type` (detached → 0, end-terrace → 2, mid-terrace → 3).
+    # TODO(cert→ventilation 6): `monthly_wind_speed_m_s` defaults to
+    #   Table U2 non-regional. Should select the regional row keyed by
+    #   `epc.region_code` once regional weather is wired in.
+    ventilation = ventilation_from_inputs(
         volume_m3=vol,
         storey_count=storeys,
         is_timber_or_steel_frame=_is_timber_or_steel_frame(epc.sap_building_parts),
@@ -780,10 +811,8 @@ def cert_to_inputs(
         passive_vents=vc.passive_vents,
         flueless_gas_fires=vc.flueless_gas_fires,
         window_pct_draught_proofed=float(epc.percent_draughtproofed or 0),
-        # SAP §2 shelter factor: 2 sheltered sides default per typical
-        # UK terraced/semi-detached layout. The cert doesn't lodge a
-        # sheltered-sides count, so we apply the spec's typical default.
         sheltered_sides=2,
+        mv_kind=MechanicalVentilationKind.NATURAL,
     )
 
     main = _first_main_heating(epc)
@@ -847,7 +876,9 @@ def cert_to_inputs(
     return CalculatorInputs(
         dimensions=dim,
         heat_transmission=ht,
-        infiltration_ach=infiltration.total_ach,
+        # SAP10.2 line (25)m — 12-month effective air-change rate from the
+        # full §2 worksheet (openings, shelter, wind adjustment, MV mode).
+        monthly_infiltration_ach=ventilation.effective_monthly_ach,
         region=_region_index(epc.region_code),
         windows=_window_inputs(epc.sap_windows),
         control_type=_control_type(main),

packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py

@@ -354,8 +354,10 @@ def test_open_chimneys_raise_infiltration_ach() -> None:
     inputs_base = cert_to_inputs(base)
     inputs_chim = cert_to_inputs(with_chimney)
 
-    # Assert
+    # Assert — direction check on the annual mean of (25)m.
-    assert inputs_chim.infiltration_ach > inputs_base.infiltration_ach
+    assert sum(inputs_chim.monthly_infiltration_ach) > sum(
+        inputs_base.monthly_infiltration_ach
+    )
 
 
 def test_living_area_fraction_uses_rdsap_table_27_by_habitable_rooms() -> None:

packages/domain/src/domain/sap/tests/test_bre_worked_examples.py

@@ -58,6 +58,8 @@ def _baseline_dwelling() -> CalculatorInputs:
         windows_w_per_k=25.0,
         doors_w_per_k=5.0,
         thermal_bridging_w_per_k=20.0,
+        fabric_heat_loss_w_per_k=130.0,        # 60+20+20+0+25+5
+        total_external_element_area_m2=200.0,  # synthetic placeholder
         total_w_per_k=150.0,
     )
     windows = (
@@ -81,7 +83,7 @@ def _baseline_dwelling() -> CalculatorInputs:
     return CalculatorInputs(
         dimensions=dim,
         heat_transmission=ht,
-        infiltration_ach=0.7,
+        monthly_infiltration_ach=(0.7,) * 12,
         region=0,
         windows=windows,
         control_type=2,

packages/domain/src/domain/sap/tests/test_calculator.py

@@ -54,6 +54,8 @@ def _baseline_inputs() -> CalculatorInputs:
         windows_w_per_k=25.0,
         doors_w_per_k=5.0,
         thermal_bridging_w_per_k=20.0,
+        fabric_heat_loss_w_per_k=130.0,        # 60+20+20+0+25+5
+        total_external_element_area_m2=200.0,  # synthetic placeholder
         total_w_per_k=150.0,
     )
     windows = (
@@ -77,7 +79,7 @@ def _baseline_inputs() -> CalculatorInputs:
     return CalculatorInputs(
         dimensions=dim,
         heat_transmission=ht,
-        infiltration_ach=0.7,
+        monthly_infiltration_ach=(0.7,) * 12,
         region=0,
         windows=windows,
         control_type=2,
@@ -170,8 +172,9 @@ def test_calculate_exposes_ventilation_intermediates() -> None:
     result = calculate_sap_from_inputs(inputs)
 
     # Assert
-    assert result.intermediate["infiltration_ach"] == inputs.infiltration_ach
+    annual_mean_ach = sum(inputs.monthly_infiltration_ach) / 12.0
-    expected_hlc_v = inputs.infiltration_ach * inputs.dimensions.volume_m3 * 0.33
+    assert result.intermediate["infiltration_ach"] == pytest.approx(annual_mean_ach, rel=1e-12)
+    expected_hlc_v = annual_mean_ach * inputs.dimensions.volume_m3 * 0.33
     assert result.intermediate["infiltration_w_per_k"] == pytest.approx(
         expected_hlc_v, rel=1e-9
     )
@@ -189,9 +192,10 @@ def test_calculate_exposes_hlc_hlp_and_annual_averages() -> None:
     result = calculate_sap_from_inputs(inputs)
 
     # Assert
+    annual_mean_ach = sum(inputs.monthly_infiltration_ach) / 12.0
     expected_hlc = (
         inputs.heat_transmission.total_w_per_k
-        + inputs.infiltration_ach * inputs.dimensions.volume_m3 * 0.33
+        + annual_mean_ach * inputs.dimensions.volume_m3 * 0.33
     )
     expected_hlp = expected_hlc / inputs.dimensions.total_floor_area_m2
     assert result.intermediate["heat_transfer_coefficient_w_per_k"] == pytest.approx(
@@ -483,8 +487,8 @@ def test_zero_heat_transmission_collapses_space_heating_to_zero() -> None:
     base = _baseline_inputs()
     no_loss = replace(
         base,
-        heat_transmission=HeatTransmission(0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0),
+        heat_transmission=HeatTransmission(0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0),
-        infiltration_ach=0.0,
+        monthly_infiltration_ach=(0.0,) * 12,
     )
 
     # Act