diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 1f11895a..72bd667b 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2946,6 +2946,17 @@ def _sap_17_1_building_part(
     perimeter_per_storey = exposed_area / total_height
     party_length_per_storey = party_area / total_height
 
+    # 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(