diff --git a/datatypes/epc/domain/epc_property_data.py b/datatypes/epc/domain/epc_property_data.py
index 94bc539e..3630b482 100644
--- a/datatypes/epc/domain/epc_property_data.py
+++ b/datatypes/epc/domain/epc_property_data.py
@@ -250,6 +250,12 @@ class SapFloorDimension:
     floor: Optional[int] = None
     floor_insulation: Optional[int] = None
     floor_construction: Optional[int] = None
+    # RdSAP10 §5.13 Table 20: True when this floor is open to outside air
+    # (exposed) or sits over enclosed unheated space (semi-exposed) — e.g.
+    # the lowest floor of an extension that hangs off the main from the
+    # first storey upward. False means a ground floor (on soil), the
+    # default path through the BS EN ISO 13370 / Table 19 cascade.
+    is_exposed_floor: bool = False
 
 
 @dataclass
diff --git a/packages/domain/src/domain/sap/worksheet/heat_transmission.py b/packages/domain/src/domain/sap/worksheet/heat_transmission.py
index ab75b0c4..eb68138f 100644
--- a/packages/domain/src/domain/sap/worksheet/heat_transmission.py
+++ b/packages/domain/src/domain/sap/worksheet/heat_transmission.py
@@ -52,6 +52,7 @@ from domain.ml.rdsap_uvalues import (
     u_basement_floor,
     u_basement_wall,
     u_door,
+    u_exposed_floor,
     u_floor,
     u_party_wall,
     u_roof,
@@ -273,11 +274,20 @@ def heat_transmission_from_cert(
                 wall_insulation_type=wall_ins_type,
             )
         ur = u_roof(country=country, age_band=age_band, insulation_thickness_mm=roof_thickness, description=roof_description)
-        # When the part carries a basement, the WHOLE floor=0 is the
-        # basement floor (per user-confirmed convention). Table 23 F-column
-        # overrides the regular floor U-value cascade.
+        # Floor U-value routing (in priority order):
+        #   1. Basement floor — Table 23 F-column override (whole floor=0).
+        #   2. Exposed/semi-exposed upper floor — Table 20 lookup; no
+        #      geometry input. Set on the ground SapFloorDimension when
+        #      the part hangs off the main from the first storey upward
+        #      (e.g. 000490 Extension 1).
+        #   3. Ground floor — BS EN ISO 13370 / Table 19 cascade.
+        is_exposed_floor = bool(ground_fd is not None and ground_fd.is_exposed_floor)
         if part.has_basement:
             uf = u_basement_floor(age_band)
+        elif is_exposed_floor:
+            uf = u_exposed_floor(
+                age_band=age_band, insulation_thickness_mm=floor_ins_thickness
+            )
         else:
             uf = u_floor(
                 country=country, age_band=age_band, construction=floor_construction,
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py b/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py
index e50969d1..3badb7d7 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py
@@ -80,6 +80,45 @@ def test_roof_insulated_assumed_with_ni_thickness_uses_50mm_per_section_5_11_4()
     assert result.roof_w_per_k == pytest.approx(68.0, abs=2.0)
 
 
+def test_exposed_timber_floor_age_b_uses_table_20_u_120_not_iso_13370() -> None:
+    # Arrange — RdSAP10 §5.13 Table 20: a part whose lowest floor sits
+    # over outside air (or unheated space) rather than soil takes its
+    # U-value from Table 20, not the BS EN ISO 13370 ground-floor branch.
+    # Elmhurst worksheet 000490 Extension 1 is exactly this shape — the
+    # extension hangs off the main from the first storey upward, so its
+    # floor=0 is "exposed timber" at U=1.20 W/m²K. Without the routing,
+    # the cascade would treat the same dimensions as a small ground-floor
+    # rectangle and return a much lower U via ISO 13370.
+    # Geometry: 18 m² × 8.68 m perimeter at age B, no insulation lodged.
+    # floor_w_per_k expected = 1.20 × 18 = 21.6 W/K.
+    main = make_building_part(
+        identifier="Main Dwelling",
+        construction_age_band="B",
+        wall_construction=4,
+        wall_insulation_type=4,
+        party_wall_construction=1,
+        roof_construction=4,
+        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,
+            ),
+        ],
+    )
+    main.sap_floor_dimensions[0].is_exposed_floor = True
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=18.0,
+        country_code="ENG",
+        sap_building_parts=[main],
+    )
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert
+    assert result.floor_w_per_k == pytest.approx(21.6, abs=0.1)
+
+
 def test_floor_insulated_assumed_with_ni_thickness_uses_50mm_per_table19_footnote() -> None:
     # Arrange — 2 413 corpus certs lodge floors with thickness="NI" and
     # description "Solid, insulated (assumed)". The retrofit-insulation