diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 5b127006..55942f4a 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -678,7 +678,10 @@ class EpcPropertyDataMapper:
             extensions_count=0,
             heated_rooms_count=0,
             open_chimneys_count=0,
-            habitable_rooms_count=0,
+            # D3: full SAP measures living_area; the engine reads it only via
+            # habitable_rooms_count (Table 27). Back-solve the count whose
+            # Table-27 fraction best matches the measured living_area/TFA.
+            habitable_rooms_count=_sap_back_solved_habitable_rooms(schema),
             # D2: door openings (1/2/3) → counts + area-weighted U. New-build
             # doors are treated insulated, so insulated_door_count == door_count.
             door_count=door_count,
@@ -2455,6 +2458,30 @@ class EpcPropertyDataMapper:
 # ---------------------------------------------------------------------------
 
 
+# RdSAP 10 Table 27 (p.52) living-area fraction by habitable-room count.
+# Mirrored here read-only to back-solve a room count from full SAP's measured
+# living_area (single home is the calculator; this is the inverse lookup).
+_SAP_LIVING_AREA_FRACTION_BY_ROOMS: Final[Dict[int, float]] = {
+    1: 0.75, 2: 0.50, 3: 0.30, 4: 0.25, 5: 0.21, 6: 0.18, 7: 0.16, 8: 0.14,
+}
+
+
+def _sap_back_solved_habitable_rooms(schema: SapSchema17_1) -> int:
+    """D3: pick the habitable-room count whose Table 27 fraction is closest to
+    the measured living_area/total_floor_area, so the engine's Table-27 path
+    reproduces the measured living-area fraction. Falls back to 0 (engine's
+    0.21 SAP-convention default) when living_area or floor area is absent."""
+    if not schema.living_area or not schema.total_floor_area:
+        return 0
+    measured_fraction = float(schema.living_area) / float(schema.total_floor_area)
+    return min(
+        _SAP_LIVING_AREA_FRACTION_BY_ROOMS,
+        key=lambda rooms: abs(
+            _SAP_LIVING_AREA_FRACTION_BY_ROOMS[rooms] - measured_fraction
+        ),
+    )
+
+
 def _sap_17_1_building_part(
     bp: SapBuildingPart_SAP_17_1, index: int
 ) -> SapBuildingPart:
diff --git a/datatypes/epc/domain/tests/test_from_sap_schema.py b/datatypes/epc/domain/tests/test_from_sap_schema.py
index a5413a2f..4ccfd756 100644
--- a/datatypes/epc/domain/tests/test_from_sap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_sap_schema.py
@@ -209,6 +209,29 @@ class TestFromSapSchema17_1UnknownOpeningType:
             EpcPropertyDataMapper.from_sap_schema_17_1(schema)
 
 
+class TestFromSapSchema17_1LivingArea:
+    """Slice 6 (D3): full SAP measures living_area but the engine only reads it
+    via habitable_rooms_count (Table 27). Back-solve the room count whose Table
+    27 fraction is closest to the measured living_area/TFA so the existing
+    engine path reproduces the measured living-area fraction."""
+
+    def _map(self, fixture: str) -> EpcPropertyData:
+        schema = from_dict(SapSchema17_1, load(fixture))
+        return EpcPropertyDataMapper.from_sap_schema_17_1(schema)
+
+    def test_sample_back_solves_rooms(self) -> None:
+        # 23.45 / 68 = 0.345 → closest Table 27 fraction is 3 rooms (0.30).
+        assert self._map("sap_17_1.json").habitable_rooms_count == 3
+
+    def test_low_living_fraction_back_solves_high_room_count(self) -> None:
+        # 15.19 / 114 = 0.133 → bottom of Table 27 → 8 rooms.
+        assert self._map("sap_17_1_house.json").habitable_rooms_count == 8
+
+    def test_flat_back_solves_rooms(self) -> None:
+        # 14.87 / 41 = 0.363 → 3 rooms (0.30 closest).
+        assert self._map("sap_17_1_flat.json").habitable_rooms_count == 3
+
+
 class TestFromSapSchema17_1Perimeter:
     """Slice 5 (D1): full SAP lodges no heat-loss perimeter; derive it from the
     measured exposed-wall areas (wall_type 1/2/3) ÷ Σ storey-heights, with party
diff --git a/datatypes/epc/schema/sap_schema_17_1.py b/datatypes/epc/schema/sap_schema_17_1.py
index e26de635..d80ee557 100644
--- a/datatypes/epc/schema/sap_schema_17_1.py
+++ b/datatypes/epc/schema/sap_schema_17_1.py
@@ -121,3 +121,6 @@ class SapSchema17_1:
     floors: List[EnergyElement]
     sap_opening_types: List[SapOpeningType]
     sap_building_parts: List[SapBuildingPart]
+    # measured living-room area (m²); the engine consumes it via a back-solved
+    # habitable_rooms_count (Table 27). Optional — 100% present in the corpus.
+    living_area: Optional[Union[int, float]] = None