diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 4f6abc04..c79bb76b 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2373,9 +2373,22 @@ def _api_party_wall_construction_int(value: Union[int, str, None]) -> Optional[i
 # 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.
+# Code 3 = "suspended, not timber" (e.g. beam-and-block / suspended
+# concrete). RdSAP 10 field 3-1 "Floor construction" enumerates the
+# lowest-floor construction as solid / suspended timber / suspended,
+# not timber. The timber/not-timber split is load-bearing: the spec's
+# "Suspended not timber (structural infiltration 0)" means only
+# "Suspended timber" triggers the §5 (12) 0.1/0.2 floor-infiltration
+# adjustment (see `_has_suspended_timber_floor_per_spec`), while a
+# not-timber suspended floor is infiltration 0. Mapping to the canonical
+# "Suspended, not timber" string (also used by the site-notes mapper)
+# takes the suspended U-value branch via the "Suspended" prefix yet
+# correctly fails the exact-match timber gate. Observed on 53/1000 of a
+# random 2026 API sample (was raising UnmappedApiCode, blocking the cert).
 _API_FLOOR_CONSTRUCTION_TO_STR: Dict[int, str] = {
     1: "Solid",
     2: "Suspended timber",
+    3: "Suspended, not timber",
     4: "Solid",
 }
 
diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index 58b9ed1a..c3d59891 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -795,3 +795,38 @@ class TestElmhurstGlazingTypeWrappedGap:
 
         # Assert
         assert code == 2
+
+
+class TestApiFloorConstructionCode:
+    """`_api_floor_construction_str` maps the GOV.UK API integer
+    floor_construction code to the description string the cascade's
+    `u_floor` + the §5 (12) infiltration rule read. RdSAP 10 field 3-1
+    "Floor construction" enumerates the lowest-floor construction as one
+    of: solid / suspended timber / suspended, not timber. The spec's
+    "Suspended not timber (structural infiltration 0)" makes the
+    timber/not-timber split load-bearing: only "Suspended timber"
+    triggers the §5 (12) 0.1/0.2 floor-infiltration adjustment;
+    "suspended, not timber" is structural-infiltration 0."""
+
+    def test_code_3_maps_to_suspended_not_timber(self) -> None:
+        # Arrange
+        from datatypes.epc.domain.mapper import _api_floor_construction_str  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_floor_construction_str(3)
+
+        # Assert — suspended U-value branch fires (starts "Suspended"),
+        # but the exact-match "Suspended timber" (12) rule does NOT —
+        # per RdSAP 10 "suspended not timber (structural infiltration 0)".
+        # Same canonical string the site-notes mapper already uses.
+        assert result == "Suspended, not timber"
+
+    def test_code_2_still_maps_to_suspended_timber(self) -> None:
+        # Arrange — regression guard: the timber code is unchanged.
+        from datatypes.epc.domain.mapper import _api_floor_construction_str  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_floor_construction_str(2)
+
+        # Assert
+        assert result == "Suspended timber"