diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index 789483e6c..0bc92b9c4 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -1082,9 +1082,39 @@ def heat_transmission_from_cert(
         is_above_partial = bool(
             ground_fd is not None and ground_fd.is_above_partially_heated_space
         )
+        # RdSAP 10 §3.12 (PDF p.26) — the FLATS AND MAISONETTES floor rules:
+        # "There is no heat loss through the floor if there is another flat
+        # below. Otherwise the floor area of the flat, or the lower floor of
+        # the maisonette, is: ... a semi-exposed floor if there are unheated
+        # premises below it (e.g. an enclosed garage) ... Semi-exposed
+        # (sheltered) floors are treated as if they were fully exposed".
+        # §5.13 (PDF p.48) confirms the U-value identity: "no distinction is
+        # made ... between an exposed floor ... and a semi-exposed floor ...
+        # and the U-values in Table 20 are used".
+        #
+        # API floor_heat_loss=2 -> floor_type "To unheated space". It set NO
+        # exposure signal, so on a mid-/top-floor flat (where
+        # `_dwelling_exposure_from_type` defaults has_exposed_floor=False,
+        # assuming a heated dwelling below) nothing overrode the suppression
+        # and the entire floor was billed at 0 W/K.
+        #
+        # SCOPED TO THE FLAT CASE ON PURPOSE. §3.12 is the flats/maisonettes
+        # section, and the accredited Elmhurst worksheet disagrees with Table 20
+        # for a HOUSE: golden cert 7536-3827-0600-0600-0276 (detached) lodges
+        # floor_heat_loss=2 on its Main part and the worksheet bills that floor
+        # at the BS EN ISO 13370 ground-floor U 0.97, NOT Table 20's 1.20
+        # (worksheet-validated by simulated case 15). A house's floor stays on
+        # the ground-floor cascade; only a flat whose floor would otherwise be
+        # suppressed to zero takes the §3.12 semi-exposed route.
+        part_floor_is_unheated_space = (
+            "to unheated space" in (part.floor_type or "").lower()
+        )
+        semi_exposed_flat_floor = (
+            part_floor_is_unheated_space and not exposure.has_exposed_floor
+        )
         if part.has_basement:
             uf = u_basement_floor(age_band)
-        elif is_exposed_floor:
+        elif is_exposed_floor or semi_exposed_flat_floor:
             uf = u_exposed_floor(
                 age_band=age_band, insulation_thickness_mm=floor_ins_thickness
             )
@@ -1228,7 +1258,7 @@ def heat_transmission_from_cert(
         # the "another dwelling below" party signal overrides it downward.
         part_has_exposed_floor = (
             exposure.has_exposed_floor or is_exposed_floor or is_above_partial
-            or part_floor_is_ground
+            or part_floor_is_ground or semi_exposed_flat_floor
         ) and not part_floor_is_party
         floor_area_total = _round_half_up(
             geom["ground_floor_area_m2"] if part_has_exposed_floor else 0.0,
diff --git a/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py b/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
index 2df247a2d..cf6fa0bca 100644
--- a/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
+++ b/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
@@ -2713,3 +2713,84 @@ def test_room_in_roof_detailed_gable_wall_excluded_from_line_31_external_area()
     assert result.total_external_element_area_m2 == pytest.approx(
         expected_line_31, abs=0.001
     )
+
+
+def test_floor_to_unheated_space_on_flat_carries_semi_exposed_loss() -> None:
+    # Arrange — a mid-/top-floor flat whose lowest floor is lodged "To unheated
+    # space" (API floor_heat_loss=2, e.g. an enclosed garage or undercroft
+    # below). RdSAP 10 §3.12 (PDF p.26) — the flats/maisonettes section — makes
+    # this a SEMI-exposed floor: "There is no heat loss through the floor if
+    # there is another flat below. Otherwise the floor area of the flat ... is
+    # ... a semi-exposed floor if there are unheated premises below it (e.g. an
+    # enclosed garage) ... Semi-exposed (sheltered) floors are treated as if
+    # they were fully exposed". §5.13 (PDF p.48) confirms the U-value identity:
+    # "no distinction is made ... between an exposed floor ... and a
+    # semi-exposed floor ... and the U-values in Table 20 are used".
+    #
+    # Code 2 previously set NO exposure signal, so the dwelling-level flat
+    # heuristic (has_exposed_floor=False, assuming a heated dwelling below) went
+    # unopposed and the whole floor was billed at 0 W/K.
+    main = make_building_part(
+        construction_age_band="B",
+        wall_construction=4, wall_insulation_type=4,
+        party_wall_construction=1, roof_construction=4,
+        floor_type="To unheated space",
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=18.0, room_height_m=2.88,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=8.68, floor=0,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=18.0, country_code="ENG", sap_building_parts=[main],
+    )
+
+    # Act — dwelling-level exposure suppresses the floor (flat label).
+    result = heat_transmission_from_cert(
+        epc, exposure=DwellingExposure(has_exposed_floor=False, has_exposed_roof=True),
+    )
+
+    # Assert — Table 20 semi-exposed loss (1.20 W/m²K × 18 m² = 21.6 W/K),
+    # not the suppressed 0.0.
+    assert result.floor_w_per_k == pytest.approx(21.6, abs=0.1)
+
+
+def test_floor_to_unheated_space_on_house_stays_on_the_ground_floor_cascade() -> None:
+    # Arrange — the SAME lodgement on a dwelling whose floor is already counted
+    # (a house: has_exposed_floor=True). §3.12's semi-exposed rule is scoped to
+    # flats and maisonettes, and the accredited Elmhurst worksheet agrees for a
+    # house: golden cert 7536-3827-0600-0600-0276 (detached) lodges
+    # floor_heat_loss=2 on its Main part and the worksheet bills that floor on
+    # the BS EN ISO 13370 ground-floor cascade at U 0.97 — NOT Table 20's 1.20
+    # (worksheet-validated by simulated case 15).
+    #
+    # So a house lodging "To unheated space" must bill exactly as if it lodged a
+    # ground floor. This pins the scope of the flat fix above.
+    def _house(floor_type: str) -> float:
+        part = make_building_part(
+            construction_age_band="B",
+            wall_construction=4, wall_insulation_type=4,
+            party_wall_construction=1, roof_construction=4,
+            floor_type=floor_type,
+            floor_dimensions=[
+                make_floor_dimension(
+                    total_floor_area_m2=18.0, room_height_m=2.88,
+                    party_wall_length_m=0.0, heat_loss_perimeter_m=8.68, floor=0,
+                ),
+            ],
+        )
+        epc = make_minimal_sap10_epc(
+            total_floor_area_m2=18.0, country_code="ENG", sap_building_parts=[part],
+        )
+        return heat_transmission_from_cert(
+            epc,
+            exposure=DwellingExposure(has_exposed_floor=True, has_exposed_roof=True),
+        ).floor_w_per_k
+
+    # Act
+    unheated_space = _house("To unheated space")
+
+    # Assert — identical to the ground-floor lodgement, i.e. untouched by the
+    # §3.12 flat rule (and in particular NOT Table 20's 1.20 × 18 = 21.6).
+    assert unheated_space == pytest.approx(_house("Ground floor"), abs=1e-9)