Raise ValueError when floor dims absent and synthesis guards cannot fire 🟩

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

datatypes/epc/domain/mapper.py

@@ -2968,6 +2968,12 @@ def _sap_17_1_building_part(
             )
         ]
     else:
+        if not bp.sap_floor_dimensions:
+            raise ValueError(
+                f"building_part {bp.building_part_number!r}: sap_floor_dimensions "
+                f"is empty and cannot be synthesised (is_single_part={is_single_part}, "
+                f"is_flat={is_flat})"
+            )
         floor_dimensions = [
             SapFloorDimension(
                 room_height_m=fd.storey_height,

datatypes/epc/domain/tests/test_from_sap_schema.py

@@ -817,3 +817,15 @@ class TestFullSapSchema16xNoFloorDimensions:
 
         # Act / Assert
         assert fd.heat_loss_perimeter_m == pytest.approx(47.08)
+
+    def test_raises_when_floor_dims_absent_and_synthesis_not_possible(
+        self,
+    ) -> None:
+        # Arrange: strip sap_flat_details so is_flat=False — synthesis guard
+        # cannot fire, empty floor dims must raise rather than silently produce [].
+        data = load("sap_16_0_full_no_floor_dims.json")
+        data.pop("sap_flat_details", None)
+
+        # Act / Assert
+        with pytest.raises(ValueError, match="sap_floor_dimensions"):
+            EpcPropertyDataMapper.from_api_response(data)