diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 0f1b8bad..1f11895a 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -799,7 +799,13 @@ 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,
+                )
                 for i, bp in enumerate(schema.sap_building_parts)
             ],
             # D6: full-SAP heating — translate the differing field names onto
@@ -2903,7 +2909,11 @@ 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,
 ) -> SapBuildingPart:
     """D1: build one `SapBuildingPart`, deriving the heat-loss perimeter and
     party-wall length the engine needs from the measured wall areas full SAP
@@ -2913,6 +2923,12 @@ 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
@@ -2930,17 +2946,28 @@ 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
-    ]
+    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:
+        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 42bb1867..2153b3f5 100644
--- a/datatypes/epc/domain/tests/test_from_sap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_sap_schema.py
@@ -777,14 +777,14 @@ class TestFullSapSchema16xNoFloorDimensions:
         assert epc.uprn == 10090783001
         assert epc.total_floor_area_m2 == 72.0
 
-    def test_building_part_has_empty_floor_dimensions(
+    def test_building_part_floor_dimensions_synthesised(
         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.
+        # 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 main_part.sap_floor_dimensions == []
+        assert len(main_part.sap_floor_dimensions) == 1
 
     def test_total_floor_area_available_at_cert_level(
         self, epc: EpcPropertyData
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