diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index bc8c7957..bb21b897 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -799,7 +799,15 @@ class EpcPropertyDataMapper:
             # D1: derive heat-loss perimeter + party-wall length from the
             # measured wall areas (full SAP lodges neither directly).
             sap_building_parts=[
-                _sap_17_1_building_part(bp, i)
+                _sap_17_1_building_part(
+                    bp,
+                    i,
+                    total_floor_area=float(schema.total_floor_area),
+                    is_single_part=len(schema.sap_building_parts) == 1,
+                    is_flat=schema.sap_flat_details is not None,
+                    address_line_1=schema.address_line_1,
+                    postcode=schema.postcode,
+                )
                 for i, bp in enumerate(schema.sap_building_parts)
             ],
             # D6: full-SAP heating — translate the differing field names onto
@@ -2905,7 +2913,13 @@ def _with_recorded_performance(
 
 
 def _sap_17_1_building_part(
-    bp: SapBuildingPart_SAP_17_1, index: int
+    bp: SapBuildingPart_SAP_17_1,
+    index: int,
+    total_floor_area: float,
+    is_single_part: bool,
+    is_flat: bool,
+    address_line_1: str,
+    postcode: str,
 ) -> SapBuildingPart:
     """D1: build one `SapBuildingPart`, deriving the heat-loss perimeter and
     party-wall length the engine needs from the measured wall areas full SAP
@@ -2915,6 +2929,12 @@ def _sap_17_1_building_part(
     length, 5 internal → discarded; any other code fails loud. Exposed/party
     areas divide by Σ storey-heights so the per-storey perimeter, summed as
     Σ(perimeter × height) by the engine, reconstructs the measured area exactly.
+
+    When `bp.sap_floor_dimensions` is absent (SAP-16.0 flat certs) and the cert
+    is confirmed single-part and flat-type, synthesises one SapFloorDimension
+    from the cert-level `total_floor_area`. Only safe for single-storey
+    single-part flats; multi-part or multi-storey cases are left empty so
+    downstream failures stay diagnosable.
     """
     exposed_area = 0.0
     party_area = 0.0
@@ -2932,17 +2952,46 @@ def _sap_17_1_building_part(
     perimeter_per_storey = exposed_area / total_height
     party_length_per_storey = party_area / total_height
 
-    floor_dimensions = [
-        SapFloorDimension(
-            room_height_m=fd.storey_height,
-            total_floor_area_m2=fd.total_floor_area,
-            heat_loss_perimeter_m=perimeter_per_storey,
-            party_wall_length_m=party_length_per_storey,
-            floor=fd.storey,
-            floor_construction=fd.floor_type,
-        )
-        for fd in bp.sap_floor_dimensions
-    ]
+    # Guard: synthesise only when floor dims are absent AND we can confirm
+    # single-storey single-part. `is_flat` (sap_flat_details present) is the
+    # current signal — flats are always single-storey within the dwelling.
+    # Bungalows are equally single-storey but carry no equivalent structural
+    # field; `dwelling_type` string matching is the only handle, and has not
+    # been confirmed necessary (no bungalows with empty floor dims were found
+    # in the failing batch). An alternative worth considering: drop `is_flat`
+    # entirely and trigger on `not bp.sap_floor_dimensions and is_single_part`
+    # alone — if the cert lodged no dims at all, there is no per-storey data to
+    # misattribute regardless of dwelling type. Requires corpus evidence that
+    # multi-storey single-part certs never omit floor dims before widening.
+    if not bp.sap_floor_dimensions and is_single_part and is_flat:
+        floor_dimensions = [
+            SapFloorDimension(
+                room_height_m=1.0,
+                total_floor_area_m2=total_floor_area,
+                heat_loss_perimeter_m=perimeter_per_storey,
+                party_wall_length_m=party_length_per_storey,
+                floor=0,
+            )
+        ]
+    else:
+        if not bp.sap_floor_dimensions:
+            raise ValueError(
+                f"{address_line_1!r}, {postcode!r}, "
+                f"building_part {bp.building_part_number!r}: "
+                f"sap_floor_dimensions is empty and cannot be synthesised "
+                f"(is_single_part={is_single_part}, is_flat={is_flat})"
+            )
+        floor_dimensions = [
+            SapFloorDimension(
+                room_height_m=fd.storey_height,
+                total_floor_area_m2=fd.total_floor_area,
+                heat_loss_perimeter_m=perimeter_per_storey,
+                party_wall_length_m=party_length_per_storey,
+                floor=fd.storey,
+                floor_construction=fd.floor_type,
+            )
+            for fd in bp.sap_floor_dimensions
+        ]
 
     if index == 0:
         identifier = BuildingPartIdentifier.MAIN
diff --git a/datatypes/epc/domain/tests/test_from_sap_schema.py b/datatypes/epc/domain/tests/test_from_sap_schema.py
index 25acf566..d05b7d78 100644
--- a/datatypes/epc/domain/tests/test_from_sap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_sap_schema.py
@@ -751,3 +751,81 @@ 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_floor_dimensions_synthesised(
+        self, epc: EpcPropertyData
+    ) -> None:
+        # The API lodges no sap_floor_dimensions for this cert family, but the
+        # mapper synthesises one from the cert-level total_floor_area so that
+        # roof_area() and gross_heat_loss_wall_area() do not crash downstream.
+        main_part = epc.sap_building_parts[0]
+        assert len(main_part.sap_floor_dimensions) == 1
+
+    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
+
+    def test_synthesised_floor_dimension_total_area(self, epc: EpcPropertyData) -> None:
+        # Arrange
+        fd = epc.sap_building_parts[0].sap_floor_dimensions[0]
+
+        # Act / Assert
+        assert fd.total_floor_area_m2 == 72.0
+
+    def test_synthesised_floor_dimension_heat_loss_perimeter(
+        self, epc: EpcPropertyData
+    ) -> None:
+        # Arrange: both sap_walls are wall_type=2 (exposed): 43.2 + 3.88 = 47.08 m².
+        # total_height falls back to 1.0 (no storey heights lodged), so perimeter = 47.08 m.
+        fd = epc.sap_building_parts[0].sap_floor_dimensions[0]
+
+        # 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="Wardalls Grove.*SE14 5FB.*sap_floor_dimensions"):
+            EpcPropertyDataMapper.from_api_response(data)
diff --git a/datatypes/epc/schema/sap_schema_17_1.py b/datatypes/epc/schema/sap_schema_17_1.py
index b84d3270..03d79f2f 100644
--- a/datatypes/epc/schema/sap_schema_17_1.py
+++ b/datatypes/epc/schema/sap_schema_17_1.py
@@ -171,6 +171,11 @@ class EnergyElement:
     environmental_efficiency_rating: int
 
 
+@dataclass
+class SapFlatDetails:
+    level: Optional[int] = None
+
+
 @dataclass
 class SapSchema17_1:
     uprn: int
@@ -199,3 +204,5 @@ class SapSchema17_1:
     # 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
+    # Present for flat-type dwellings; absence means not a flat.
+    sap_flat_details: Optional[SapFlatDetails] = None