diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index c79bb76b..f8a31ae4 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2385,7 +2385,18 @@ def _api_party_wall_construction_int(value: Union[int, str, None]) -> Optional[i
 # 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] = {
+#
+# Code 0 = "not recorded / not applicable" → None. It pairs
+# overwhelmingly with floor_heat_loss=6 ("another dwelling below" — an
+# upper-floor flat with no ground floor to describe) but also appears
+# with mixed Solid / unheated-space descriptions, so there is no single
+# construction to assert. None defers to RdSAP 10 Table 19 ("where floor
+# construction is unknown" → age-band default), exactly as an unlodged
+# floor_construction does. Empirically inert: floor W/K is identical to
+# any explicit construction across all 37 code-0 certs in the 2026
+# sample (the heat loss is governed by floor_heat_loss, not this field).
+_API_FLOOR_CONSTRUCTION_TO_STR: Dict[int, Optional[str]] = {
+    0: None,
     1: "Solid",
     2: "Suspended timber",
     3: "Suspended, not timber",
diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index c3d59891..4f0b80ee 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -830,3 +830,22 @@ class TestApiFloorConstructionCode:
 
         # Assert
         assert result == "Suspended timber"
+
+    def test_code_0_maps_to_none_unknown_construction(self) -> None:
+        # Arrange — code 0 is the "not recorded / not applicable"
+        # sentinel: it pairs overwhelmingly with floor_heat_loss=6
+        # ("another dwelling below", an upper-floor flat with no ground
+        # floor to describe), but also appears with mixed Solid / unheated
+        # descriptions. There is no single construction to assert, so it
+        # maps to None — RdSAP 10 Table 19 ("where floor construction is
+        # unknown" → age-band default), the same treatment as an unlodged
+        # floor_construction. Empirically inert: floor W/K is identical to
+        # any explicit construction string across all observed code-0
+        # certs (the heat loss is governed by floor_heat_loss, not this).
+        from datatypes.epc.domain.mapper import _api_floor_construction_str  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_floor_construction_str(0)
+
+        # Assert — no raise; None defers to the cascade's Table 19 default.
+        assert result is None