diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index ee86817a..efe3a7fd 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2311,11 +2311,17 @@ def _api_party_wall_construction_int(value: Union[int, str, None]) -> Optional[i
 # cascade's `u_floor` looks for via the "Suspended"/"Solid" prefix
 # (see Slice 88 — `heat_transmission.py` consumes `bp.floor_
 # construction_type` to choose the suspended-branch BS EN ISO 13370
-# formula). Only the values observed across the 10 golden fixtures
-# (1, 2) are mapped; unrecognised codes fall through to None.
+# formula). Code 4 observed on cert 0712 (basement smoke-test
+# fixture): paired with `floor_insulation=0` and global floor
+# descriptions "Solid" + "Solid, no insulation (assumed)" — semantically
+# a solid floor with the no-insulation variant. The cascade only
+# distinguishes "Suspended" vs everything-else (the solid-branch is
+# the default fall-through), so the additional code maps to the same
+# "Solid" string as code 1.
 _API_FLOOR_CONSTRUCTION_TO_STR: Dict[int, str] = {
     1: "Solid",
     2: "Suspended timber",
+    4: "Solid",
 }