diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 1658383f..7fe948dc 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -735,7 +735,11 @@ class EpcPropertyDataMapper:
             postcode=schema.postcode,
             post_town=schema.post_town,
             total_floor_area_m2=float(schema.total_floor_area),
-            has_hot_water_cylinder=schema.has_hot_water_cylinder == "true",
+            has_hot_water_cylinder=(
+                schema.has_hot_water_cylinder
+                or schema.sap_heating.has_hot_water_cylinder
+            )
+            == "true",
             has_fixed_air_conditioning=schema.has_fixed_air_conditioning == "true",
             solar_water_heating=False,
             extensions_count=0,
diff --git a/datatypes/epc/domain/tests/test_from_sap_schema.py b/datatypes/epc/domain/tests/test_from_sap_schema.py
index 4cc08213..c8f6716a 100644
--- a/datatypes/epc/domain/tests/test_from_sap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_sap_schema.py
@@ -77,6 +77,39 @@ class TestFromSapSchema17_1Tracer:
         assert result.total_floor_area_m2 == 68.0
 
 
+class TestFullSapHasHotWaterCylinderFallback:
+    """Some full-SAP certs (e.g. SAP-Schema-17.0 cert 8265-7433-3220-9736-7902)
+    omit the top-level `has_hot_water_cylinder` and lodge it only under
+    `sap_heating`. The required top-level field made `from_dict` raise; it is now
+    optional and the mapper falls back to the nested value."""
+
+    def test_top_level_omitted_falls_back_to_sap_heating(self) -> None:
+        # Arrange — drop the top-level flag, keep the nested one ("true").
+        data = load("sap_17_1.json")
+        data.pop("has_hot_water_cylinder", None)
+        data["sap_heating"]["has_hot_water_cylinder"] = "true"
+
+        # Act
+        schema = from_dict(SapSchema17_1, data)
+        result = EpcPropertyDataMapper.from_sap_schema_17_1(schema)
+
+        # Assert — resolved from sap_heating, not crashed
+        assert result.has_hot_water_cylinder is True
+
+    def test_top_level_omitted_nested_false_resolves_false(self) -> None:
+        # Arrange
+        data = load("sap_17_1.json")
+        data.pop("has_hot_water_cylinder", None)
+        data["sap_heating"]["has_hot_water_cylinder"] = "false"
+
+        # Act
+        schema = from_dict(SapSchema17_1, data)
+        result = EpcPropertyDataMapper.from_sap_schema_17_1(schema)
+
+        # Assert
+        assert result.has_hot_water_cylinder is False
+
+
 class TestFromSapSchema17_1FabricDescriptions:
     """Slice 3 (D4): the measured-U fabric descriptions flow through to
     epc.walls/floors/roofs so the engine's u_wall/u_floor/u_roof can parse
diff --git a/datatypes/epc/schema/sap_schema_17_1.py b/datatypes/epc/schema/sap_schema_17_1.py
index 195e4f53..b84d3270 100644
--- a/datatypes/epc/schema/sap_schema_17_1.py
+++ b/datatypes/epc/schema/sap_schema_17_1.py
@@ -184,7 +184,6 @@ class SapSchema17_1:
     postcode: str
     post_town: str
     inspection_date: str
-    has_hot_water_cylinder: str
     has_fixed_air_conditioning: str
     roofs: List[EnergyElement]
     walls: List[EnergyElement]
@@ -197,3 +196,6 @@ class SapSchema17_1:
     # 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
+    # Some 17.0 full-SAP certs omit the top-level flag and lodge it only under
+    # sap_heating; the mapper falls back to sap_heating.has_hot_water_cylinder.
+    has_hot_water_cylinder: Optional[str] = None