Mapper synthesises SapFloorDimension for SAP-16.0 flats with empty floor dims 🟥

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

datatypes/epc/domain/tests/test_from_sap_schema.py

@@ -751,3 +751,52 @@ class TestFullSapSchema16xRouting:
 
         # Assert
         assert not any("broken schema_type" in r.message for r in caplog.records)
+
+
+class TestFullSapSchema16xNoFloorDimensions:
+    """SAP-Schema-16.0 certs with assessment_type=SAP that lodge no
+    sap_floor_dimensions inside sap_building_parts (uprn 10090783001,
+    cert lodged 88). The API only carries total_floor_area at the top level;
+    the building part has sap_walls but no per-storey dimension data.
+
+    This is the root cause of the `max() arg is an empty sequence` crash in
+    building_geometry.roof_area() — documented in
+    scripts/handover_max_empty_sequence_design_question.md.
+    """
+
+    @pytest.fixture
+    def epc(self) -> EpcPropertyData:
+        return EpcPropertyDataMapper.from_api_response(
+            load("sap_16_0_full_no_floor_dims.json")
+        )
+
+    def test_maps_successfully(self, epc: EpcPropertyData) -> None:
+        assert isinstance(epc, EpcPropertyData)
+
+    def test_uprn_and_total_floor_area(self, epc: EpcPropertyData) -> None:
+        assert epc.uprn == 10090783001
+        assert epc.total_floor_area_m2 == 72.0
+
+    def test_building_part_has_empty_floor_dimensions(
+        self, epc: EpcPropertyData
+    ) -> None:
+        # The API lodges no sap_floor_dimensions for this cert family.
+        # The mapper faithfully produces an empty list — the data gap that
+        # causes roof_area() / gross_heat_loss_wall_area() to crash downstream.
+        main_part = epc.sap_building_parts[0]
+        assert main_part.sap_floor_dimensions == []
+
+    def test_total_floor_area_available_at_cert_level(
+        self, epc: EpcPropertyData
+    ) -> None:
+        # total_floor_area_m2 IS present at the EpcPropertyData level (72 m²).
+        # This is the candidate fallback value for roof_area() — accurate for
+        # single-storey single-part certs, an overestimate for multi-storey.
+        assert epc.total_floor_area_m2 == 72.0
+
+    def test_synthesises_single_floor_dimension(self, epc: EpcPropertyData) -> None:
+        # Arrange
+        main_part = epc.sap_building_parts[0]
+
+        # Act / Assert
+        assert len(main_part.sap_floor_dimensions) == 1