diff --git a/backend/epc_client/__init__.py b/backend/epc_client/__init__.py
new file mode 100644
index 00000000..720594f7
--- /dev/null
+++ b/backend/epc_client/__init__.py
@@ -0,0 +1,3 @@
+from backend.epc_client.client import EpcClientService, EpcSearchResult
+
+__all__ = ["EpcClientService", "EpcSearchResult"]
diff --git a/backend/epc_client/_retry.py b/backend/epc_client/_retry.py
new file mode 100644
index 00000000..e290e95b
--- /dev/null
+++ b/backend/epc_client/_retry.py
@@ -0,0 +1,23 @@
+import time
+from typing import Callable, TypeVar
+
+from backend.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,
+) -> T:
+    last_exc: EpcRateLimitError | None = None
+    for attempt in range(max_retries + 1):
+        try:
+            return fn()
+        except EpcRateLimitError as exc:
+            last_exc = exc
+            if attempt < max_retries:
+                time.sleep(backoff_base * (backoff_multiplier ** attempt))
+    raise last_exc  # type: ignore[misc]
diff --git a/backend/epc_client/client.py b/backend/epc_client/client.py
new file mode 100644
index 00000000..33f25ef5
--- /dev/null
+++ b/backend/epc_client/client.py
@@ -0,0 +1,175 @@
+# Spec: https://raw.githubusercontent.com/communitiesuk/epb-data-warehouse/main/api/api.yml
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Callable, Optional
+
+import httpx
+import pandas as pd
+
+from backend.epc_client.exceptions import EpcApiError, EpcNotFoundError, EpcRateLimitError
+from backend.epc_client._retry import call_with_retry
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+
+@dataclass
+class EpcSearchResult:
+    certificate_number: str
+    address_line_1: str
+    address_line_2: Optional[str]
+    address_line_3: Optional[str]
+    address_line_4: Optional[str]
+    postcode: str
+    post_town: str
+    uprn: Optional[int]
+    current_energy_efficiency_band: str
+    registration_date: str
+
+    def full_address(self) -> str:
+        parts = [
+            self.address_line_1,
+            self.address_line_2,
+            self.address_line_3,
+            self.address_line_4,
+        ]
+        return ", ".join(p for p in parts if p)
+
+
+class EpcClientService:
+    BASE_URL = "https://api.get-energy-performance-data.communities.gov.uk"
+    _MIN_MATCH_SCORE = 0.6
+
+    def __init__(self, auth_token: str) -> None:
+        self._headers = {
+            "Authorization": f"Bearer {auth_token}",
+            "Accept": "application/json",
+        }
+
+    def get_by_certificate_number(self, cert_num: str) -> EpcPropertyData:
+        raw = call_with_retry(lambda: self._fetch_certificate(cert_num))
+        return EpcPropertyDataMapper.from_api_response(raw)
+
+    def get_by_uprn(self, uprn: int) -> Optional[EpcPropertyData]:
+        results = call_with_retry(lambda: self._search(uprn=uprn))
+        if not results:
+            return None
+        latest = max(results, key=lambda r: r.registration_date)
+        return self.get_by_certificate_number(latest.certificate_number)
+
+    def search_by_postcode(self, postcode: str) -> list[EpcSearchResult]:
+        return call_with_retry(lambda: self._search(postcode=postcode))
+
+    def find_best_match(self, postcode: str, address: str) -> Optional[EpcPropertyData]:
+        from backend.utils.addressMatch import get_uprn_candidates
+
+        candidates = self.search_by_postcode(postcode)
+        if not candidates:
+            return None
+
+        # Round 1: score on addressLine1 only
+        cert_num = self._pick_best_cert(candidates, address, use_full_address=False, fn=get_uprn_candidates)
+        if cert_num:
+            return self._safe_get(cert_num)
+
+        # Round 2: score on all address lines joined
+        cert_num = self._pick_best_cert(candidates, address, use_full_address=True, fn=get_uprn_candidates)
+        if cert_num:
+            return self._safe_get(cert_num)
+
+        return None
+
+    # ------------------------------------------------------------------
+    # Private helpers
+    # ------------------------------------------------------------------
+
+    def _fetch_certificate(self, cert_num: str) -> dict:
+        resp = httpx.get(
+            f"{self.BASE_URL}/api/certificate",
+            params={"certificate_number": cert_num},
+            headers=self._headers,
+        )
+        if resp.status_code == 404:
+            raise EpcNotFoundError(cert_num)
+        if resp.status_code == 429:
+            raise EpcRateLimitError("Rate limited by EPC API")
+        if not resp.is_success:
+            raise EpcApiError(f"EPC API error {resp.status_code}: {resp.text}")
+        return resp.json()["data"]
+
+    def _search(
+        self,
+        postcode: Optional[str] = None,
+        uprn: Optional[int] = None,
+    ) -> list[EpcSearchResult]:
+        params: dict[str, str | int] = {}
+        if postcode:
+            params["postcode"] = postcode
+        if uprn is not None:
+            params["uprn"] = uprn
+
+        resp = httpx.get(
+            f"{self.BASE_URL}/api/domestic/search",
+            params=params,
+            headers=self._headers,
+        )
+        if resp.status_code == 404:
+            return []
+        if resp.status_code == 429:
+            raise EpcRateLimitError("Rate limited by EPC API")
+        if not resp.is_success:
+            raise EpcApiError(f"EPC API error {resp.status_code}: {resp.text}")
+
+        rows = resp.json().get("data", [])
+        return [self._parse_search_result(r) for r in rows]
+
+    @staticmethod
+    def _parse_search_result(row: dict) -> EpcSearchResult:
+        return EpcSearchResult(
+            certificate_number=row["certificateNumber"],
+            address_line_1=row["addressLine1"],
+            address_line_2=row.get("addressLine2"),
+            address_line_3=row.get("addressLine3"),
+            address_line_4=row.get("addressLine4"),
+            postcode=row["postcode"],
+            post_town=row["postTown"],
+            uprn=row.get("uprn"),
+            current_energy_efficiency_band=row["currentEnergyEfficiencyBand"],
+            registration_date=row["registrationDate"],
+        )
+
+    def _pick_best_cert(
+        self,
+        candidates: list[EpcSearchResult],
+        user_address: str,
+        use_full_address: bool,
+        fn: Callable[..., pd.DataFrame],
+    ) -> Optional[str]:
+        df = pd.DataFrame([
+            {
+                "address": r.full_address() if use_full_address else r.address_line_1,
+                "uprn": str(r.uprn) if r.uprn is not None else "",
+                "certificate_number": r.certificate_number,
+            }
+            for r in candidates
+        ])
+
+        scored = fn(df, user_address=user_address)
+        if scored.empty:
+            return None
+
+        best_score = scored.iloc[0]["lexiscore"]
+        if best_score < self._MIN_MATCH_SCORE:
+            return None
+
+        top = scored[scored["lexirank"] == 1]
+        if len(top) != 1:
+            return None
+
+        return str(top.iloc[0]["certificate_number"])
+
+    def _safe_get(self, cert_num: str) -> Optional[EpcPropertyData]:
+        try:
+            return self.get_by_certificate_number(cert_num)
+        except EpcNotFoundError:
+            return None
diff --git a/backend/epc_client/exceptions.py b/backend/epc_client/exceptions.py
new file mode 100644
index 00000000..49f1542a
--- /dev/null
+++ b/backend/epc_client/exceptions.py
@@ -0,0 +1,10 @@
+class EpcApiError(Exception):
+    """Base for all EPC client errors."""
+
+
+class EpcNotFoundError(EpcApiError):
+    """Raised when the API returns 404."""
+
+
+class EpcRateLimitError(EpcApiError):
+    """Raised when the API returns 429 and all retries are exhausted."""
diff --git a/backend/epc_client/requirements.txt b/backend/epc_client/requirements.txt
new file mode 100644
index 00000000..aa69c38b
--- /dev/null
+++ b/backend/epc_client/requirements.txt
@@ -0,0 +1 @@
+httpx>=0.27.0
diff --git a/backend/epc_client/tests/__init__.py b/backend/epc_client/tests/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/backend/epc_client/tests/conftest.py b/backend/epc_client/tests/conftest.py
new file mode 100644
index 00000000..2ed444af
--- /dev/null
+++ b/backend/epc_client/tests/conftest.py
@@ -0,0 +1,48 @@
+import json
+import pathlib
+import pytest
+
+from backend.epc_client.client import EpcClientService
+
+SAMPLES_DIR = pathlib.Path("backend/epc_api/json_samples")
+
+
+@pytest.fixture
+def rdsap_21_0_0_cert():
+    return json.loads((SAMPLES_DIR / "RdSAP-Schema-21.0.0/epc.json").read_text())
+
+
+@pytest.fixture
+def rdsap_21_0_1_cert():
+    return json.loads((SAMPLES_DIR / "RdSAP-Schema-21.0.1/epc.json").read_text())
+
+
+@pytest.fixture
+def epc_service():
+    return EpcClientService(auth_token="test-token")
+
+
+def make_search_row(
+    cert_num="CERT-001",
+    address_line_1="1 Test Street",
+    postcode="SW1A 1AA",
+    post_town="London",
+    uprn=100023336956,
+    band="D",
+    registration_date="2024-01-01",
+    address_line_2=None,
+    address_line_3=None,
+    address_line_4=None,
+):
+    return {
+        "certificateNumber": cert_num,
+        "addressLine1": address_line_1,
+        "addressLine2": address_line_2,
+        "addressLine3": address_line_3,
+        "addressLine4": address_line_4,
+        "postcode": postcode,
+        "postTown": post_town,
+        "uprn": uprn,
+        "currentEnergyEfficiencyBand": band,
+        "registrationDate": registration_date,
+    }
diff --git a/backend/epc_client/tests/test_client.py b/backend/epc_client/tests/test_client.py
new file mode 100644
index 00000000..51dd2a12
--- /dev/null
+++ b/backend/epc_client/tests/test_client.py
@@ -0,0 +1,224 @@
+from unittest.mock import MagicMock, patch, call
+import pytest
+
+from backend.epc_client.client import EpcClientService, EpcSearchResult
+from backend.epc_client.exceptions import EpcNotFoundError, EpcRateLimitError
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from backend.epc_client.tests.conftest import make_search_row
+
+
+def _mock_response(status_code=200, json_data=None):
+    resp = MagicMock()
+    resp.status_code = status_code
+    resp.is_success = 200 <= status_code < 300
+    resp.json.return_value = json_data or {}
+    resp.text = str(json_data)
+    return resp
+
+
+# ---------------------------------------------------------------------------
+# Test 1: get_by_certificate_number happy path
+# ---------------------------------------------------------------------------
+
+def test_get_by_certificate_number_returns_epc_property_data(epc_service, rdsap_21_0_1_cert):
+    cert_response = {"data": rdsap_21_0_1_cert}
+    with patch("httpx.get", return_value=_mock_response(200, cert_response)):
+        result = epc_service.get_by_certificate_number("CERT-001")
+
+    assert isinstance(result, EpcPropertyData)
+
+
+# ---------------------------------------------------------------------------
+# Test 2: get_by_certificate_number 404 → EpcNotFoundError
+# ---------------------------------------------------------------------------
+
+def test_get_by_certificate_number_404_raises_not_found(epc_service):
+    with patch("httpx.get", return_value=_mock_response(404)):
+        with pytest.raises(EpcNotFoundError):
+            epc_service.get_by_certificate_number("BAD-CERT")
+
+
+# ---------------------------------------------------------------------------
+# Test 3: 429 retried, succeeds on 3rd attempt
+# ---------------------------------------------------------------------------
+
+def test_get_by_certificate_number_retries_on_429_and_succeeds(epc_service, rdsap_21_0_1_cert):
+    cert_response = {"data": rdsap_21_0_1_cert}
+    responses = [
+        _mock_response(429),
+        _mock_response(429),
+        _mock_response(200, cert_response),
+    ]
+    with patch("httpx.get", side_effect=responses), patch("time.sleep"):
+        result = epc_service.get_by_certificate_number("CERT-001")
+
+    assert isinstance(result, EpcPropertyData)
+
+
+# ---------------------------------------------------------------------------
+# Test 4: get_by_uprn empty search → None
+# ---------------------------------------------------------------------------
+
+def test_get_by_uprn_returns_none_when_no_results(epc_service):
+    with patch("httpx.get", return_value=_mock_response(200, {"data": []})):
+        result = epc_service.get_by_uprn(100023336956)
+
+    assert result is None
+
+
+# ---------------------------------------------------------------------------
+# Test 5: get_by_uprn multiple results → fetches latest by registration_date
+# ---------------------------------------------------------------------------
+
+def test_get_by_uprn_picks_most_recent_certificate(epc_service, rdsap_21_0_1_cert):
+    search_rows = [
+        make_search_row(cert_num="CERT-OLD", registration_date="2022-01-01"),
+        make_search_row(cert_num="CERT-NEW", registration_date="2024-06-01"),
+        make_search_row(cert_num="CERT-MID", registration_date="2023-03-15"),
+    ]
+    cert_response = {"data": rdsap_21_0_1_cert}
+
+    def fake_get(url, params=None, **kwargs):
+        if "search" in url:
+            return _mock_response(200, {"data": search_rows})
+        return _mock_response(200, cert_response)
+
+    with patch("httpx.get", side_effect=fake_get) as mock_get:
+        result = epc_service.get_by_uprn(100023336956)
+
+    assert isinstance(result, EpcPropertyData)
+    # Second call must be for the most recent cert
+    cert_call = mock_get.call_args_list[1]
+    assert cert_call.kwargs["params"]["certificate_number"] == "CERT-NEW"
+
+
+# ---------------------------------------------------------------------------
+# Test 6: search_by_postcode returns list[EpcSearchResult]
+# ---------------------------------------------------------------------------
+
+def test_search_by_postcode_returns_results(epc_service):
+    rows = [
+        make_search_row(cert_num="CERT-A", address_line_1="1 High Street"),
+        make_search_row(cert_num="CERT-B", address_line_1="2 High Street"),
+    ]
+    with patch("httpx.get", return_value=_mock_response(200, {"data": rows})):
+        results = epc_service.search_by_postcode("SW1A 1AA")
+
+    assert len(results) == 2
+    assert all(isinstance(r, EpcSearchResult) for r in results)
+    assert results[0].certificate_number == "CERT-A"
+    assert results[1].address_line_1 == "2 High Street"
+
+
+# ---------------------------------------------------------------------------
+# Test 7: search_by_postcode 404 → empty list
+# ---------------------------------------------------------------------------
+
+def test_search_by_postcode_404_returns_empty_list(epc_service):
+    with patch("httpx.get", return_value=_mock_response(404)):
+        results = epc_service.search_by_postcode("ZZ9 9ZZ")
+
+    assert results == []
+
+
+# ---------------------------------------------------------------------------
+# Tests 8-10: find_best_match
+# ---------------------------------------------------------------------------
+
+def _make_scored_df(rows, scores, ranks):
+    import pandas as pd
+    df = pd.DataFrame(rows)
+    df["lexiscore"] = scores
+    df["lexirank"] = ranks
+    return df.sort_values("lexirank")
+
+
+def test_find_best_match_round1_clear_winner(epc_service, rdsap_21_0_1_cert):
+    search_rows = [
+        make_search_row(cert_num="CERT-WIN", address_line_1="1 High Street"),
+        make_search_row(cert_num="CERT-LOSE", address_line_1="99 Nowhere Lane"),
+    ]
+    cert_response = {"data": rdsap_21_0_1_cert}
+
+    df_rows = [
+        {"address": "1 High Street", "uprn": "100023336956", "certificate_number": "CERT-WIN"},
+        {"address": "99 Nowhere Lane", "uprn": "100023336956", "certificate_number": "CERT-LOSE"},
+    ]
+    scored = _make_scored_df(df_rows, [0.9, 0.1], [1, 2])
+
+    def fake_get(url, params=None, **kwargs):
+        if "search" in url:
+            return _mock_response(200, {"data": search_rows})
+        return _mock_response(200, cert_response)
+
+    with patch("httpx.get", side_effect=fake_get), \
+         patch("backend.utils.addressMatch.get_uprn_candidates", return_value=scored):
+        result = epc_service.find_best_match("SW1A 1AA", "1 High Street")
+
+    assert isinstance(result, EpcPropertyData)
+
+
+def test_find_best_match_round1_ambiguous_round2_resolves(epc_service, rdsap_21_0_1_cert):
+    search_rows = [
+        make_search_row(
+            cert_num="CERT-A", address_line_1="1 High Street",
+            address_line_2="Ground Floor",
+        ),
+        make_search_row(
+            cert_num="CERT-B", address_line_1="1 High Street",
+            address_line_2="First Floor",
+        ),
+    ]
+    cert_response = {"data": rdsap_21_0_1_cert}
+
+    # Round 1: both score equally — ambiguous (two rank-1s)
+    ambiguous = _make_scored_df(
+        [
+            {"address": "1 High Street", "uprn": "111", "certificate_number": "CERT-A"},
+            {"address": "1 High Street", "uprn": "222", "certificate_number": "CERT-B"},
+        ],
+        [0.9, 0.9],
+        [1, 1],
+    )
+    # Round 2: CERT-A wins on full address
+    resolved = _make_scored_df(
+        [
+            {"address": "1 High Street, Ground Floor", "uprn": "111", "certificate_number": "CERT-A"},
+            {"address": "1 High Street, First Floor", "uprn": "222", "certificate_number": "CERT-B"},
+        ],
+        [0.85, 0.4],
+        [1, 2],
+    )
+
+    call_count = {"n": 0}
+
+    def fake_candidates(df, user_address, **kwargs):
+        call_count["n"] += 1
+        return ambiguous if call_count["n"] == 1 else resolved
+
+    def fake_get(url, params=None, **kwargs):
+        if "search" in url:
+            return _mock_response(200, {"data": search_rows})
+        return _mock_response(200, cert_response)
+
+    with patch("httpx.get", side_effect=fake_get), \
+         patch("backend.utils.addressMatch.get_uprn_candidates", side_effect=fake_candidates):
+        result = epc_service.find_best_match("SW1A 1AA", "1 High Street Ground Floor")
+
+    assert isinstance(result, EpcPropertyData)
+
+
+def test_find_best_match_returns_none_when_no_good_match(epc_service):
+    search_rows = [make_search_row(cert_num="CERT-X", address_line_1="99 Nowhere Lane")]
+
+    low_score = _make_scored_df(
+        [{"address": "99 Nowhere Lane", "uprn": "111", "certificate_number": "CERT-X"}],
+        [0.1],
+        [1],
+    )
+
+    with patch("httpx.get", return_value=_mock_response(200, {"data": search_rows})), \
+         patch("backend.utils.addressMatch.get_uprn_candidates", return_value=low_score):
+        result = epc_service.find_best_match("SW1A 1AA", "1 Completely Different Road")
+
+    assert result is None
diff --git a/backend/epc_client/tests/test_mapper_dispatcher.py b/backend/epc_client/tests/test_mapper_dispatcher.py
new file mode 100644
index 00000000..efb9c4ec
--- /dev/null
+++ b/backend/epc_client/tests/test_mapper_dispatcher.py
@@ -0,0 +1,31 @@
+import pytest
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+
+
+# ---------------------------------------------------------------------------
+# Test 1: from_api_response with RdSAP-Schema-21.0.0 fixture → EpcPropertyData
+# ---------------------------------------------------------------------------
+
+def test_from_api_response_rdsap_21_0_0(rdsap_21_0_0_cert):
+    result = EpcPropertyDataMapper.from_api_response(rdsap_21_0_0_cert)
+    assert isinstance(result, EpcPropertyData)
+
+
+# ---------------------------------------------------------------------------
+# Test 2: from_api_response with RdSAP-Schema-21.0.1 fixture → EpcPropertyData
+# ---------------------------------------------------------------------------
+
+def test_from_api_response_rdsap_21_0_1(rdsap_21_0_1_cert):
+    result = EpcPropertyDataMapper.from_api_response(rdsap_21_0_1_cert)
+    assert isinstance(result, EpcPropertyData)
+
+
+# ---------------------------------------------------------------------------
+# Test 3: unknown schema_type → ValueError
+# ---------------------------------------------------------------------------
+
+def test_from_api_response_unknown_schema_raises():
+    with pytest.raises(ValueError, match="Unsupported EPC schema"):
+        EpcPropertyDataMapper.from_api_response({"schema_type": "RdSAP-Schema-99.0.0"})
diff --git a/datatypes/epc/schema/helpers.py b/datatypes/epc/schema/helpers.py
new file mode 100644
index 00000000..22f132d2
--- /dev/null
+++ b/datatypes/epc/schema/helpers.py
@@ -0,0 +1,77 @@
+import dataclasses
+import typing
+from datetime import date
+from typing import Any, Dict, Type, TypeVar
+
+T = TypeVar("T")
+
+
+def from_dict(cls: Type[T], data: Dict[str, Any]) -> T:
+    """
+    Recursively convert a plain dict (e.g. from json.loads) into the given
+    dataclass type, using the field type hints to convert nested structures.
+
+    Handles:
+    - Nested dataclasses
+    - List[SomeDataclass]
+    - Optional[X] / Union[X, None]
+    - Union[DataclassType, primitive] (e.g. Union[Measurement, int])
+    - Primitive pass-through for Union[str, int] etc.
+    """
+    return _from_dict_impl(cls, data)  # type: ignore[return-value]
+
+
+def _from_dict_impl(cls: Any, data: Any) -> Any:
+    hints = typing.get_type_hints(cls)
+    kwargs: Dict[str, Any] = {}
+
+    for field in dataclasses.fields(cls):  # type: ignore[arg-type]
+        has_default = (
+            field.default is not dataclasses.MISSING
+            or field.default_factory is not dataclasses.MISSING  # type: ignore[misc]
+        )
+        if field.name not in data:
+            if has_default:
+                continue
+            raise ValueError(f"{cls.__name__}: missing required field '{field.name}'")
+
+        kwargs[field.name] = _coerce(data[field.name], hints[field.name])
+
+    return cls(**kwargs)
+
+
+def _coerce(value: Any, hint: Any) -> Any:
+    if value is None:
+        return None
+
+    origin = typing.get_origin(hint)
+    args = typing.get_args(hint)
+
+    # Union (includes Optional[X] which is Union[X, None])
+    if origin is typing.Union:
+        if value is None:
+            return None
+        non_none_args = [a for a in args if a is not type(None)]
+        if len(non_none_args) == 1:
+            # Optional[X] — recurse so List[X] and nested dataclasses are handled
+            return _coerce(value, non_none_args[0])
+        # Multi-type Union (e.g. Union[Measurement, int]): try dataclasses first
+        for arg in non_none_args:
+            if dataclasses.is_dataclass(arg) and isinstance(value, dict):
+                return _from_dict_impl(arg, value)
+        # All remaining args are primitives — return value as-is
+        return value
+
+    # List[X]
+    if origin is list:
+        item_hint = args[0]
+        return [_coerce(item, item_hint) for item in value]
+
+    # Plain dataclass
+    if dataclasses.is_dataclass(hint) and isinstance(value, dict):
+        return _from_dict_impl(hint, value)
+
+    if hint is date and isinstance(value, str):
+        return date.fromisoformat(value)
+
+    return value