diff --git a/packages/domain/src/domain/ml/rdsap_uvalues.py b/packages/domain/src/domain/ml/rdsap_uvalues.py
index 8ffece99..1dd5659a 100644
--- a/packages/domain/src/domain/ml/rdsap_uvalues.py
+++ b/packages/domain/src/domain/ml/rdsap_uvalues.py
@@ -587,6 +587,54 @@ def u_floor(
     return soil_g / (0.457 * b + d_t)
 
 
+# ---------------------------------------------------------------------------
+# Exposed / semi-exposed upper-floor U-values  (Table 20, §5.13)
+# ---------------------------------------------------------------------------
+#
+# Table 20 (page 47): the spec collapses exposed (to outside air) and
+# semi-exposed (to enclosed unheated space) into the same lookup. Keyed
+# on age band × insulation thickness — no geometry input. This is the
+# floor of e.g. a single-storey extension that hangs off the main from
+# the first storey upward (000490 Extension 1 is exactly this shape).
+#
+# Country footnotes:
+#   (1) Use the 50 mm row if known to be insulated but thickness unknown.
+#   (2) Band L → 0.18 W/m²K in Scotland.
+#   (3) Band M → 0.15 W/m²K in Scotland AND Wales.
+# These are England-and-Wales values; country overrides land later.
+
+_EXPOSED_FLOOR_BY_AGE_AND_INS: Final[dict[str, tuple[float, float, float, float]]] = {
+    # (unknown/as-built, 50mm, 100mm, 150mm)
+    "A": (1.20, 0.50, 0.30, 0.22), "B": (1.20, 0.50, 0.30, 0.22),
+    "C": (1.20, 0.50, 0.30, 0.22), "D": (1.20, 0.50, 0.30, 0.22),
+    "E": (1.20, 0.50, 0.30, 0.22), "F": (1.20, 0.50, 0.30, 0.22),
+    "G": (1.20, 0.50, 0.30, 0.22),
+    "H": (0.51, 0.50, 0.30, 0.22), "I": (0.51, 0.50, 0.30, 0.22),
+    "J": (0.25, 0.25, 0.25, 0.22),
+    "K": (0.22, 0.22, 0.22, 0.22),
+    "L": (0.22, 0.22, 0.22, 0.22),
+    "M": (0.18, 0.18, 0.18, 0.18),
+}
+
+
+def u_exposed_floor(
+    age_band: Optional[str], insulation_thickness_mm: Optional[int]
+) -> float:
+    """RdSAP10 Table 20 exposed/semi-exposed upper-floor U-value in W/m²K.
+    Used when the part's floor is open to outside air or sits over an
+    unheated space (e.g. an over-passageway extension) rather than over
+    soil. No geometry input — the lookup is age × insulation only."""
+    band = (age_band or "A").upper()
+    row = _EXPOSED_FLOOR_BY_AGE_AND_INS.get(band, _EXPOSED_FLOOR_BY_AGE_AND_INS["A"])
+    if insulation_thickness_mm is None or insulation_thickness_mm < 25:
+        return row[0]
+    if insulation_thickness_mm < 75:
+        return row[1]
+    if insulation_thickness_mm < 125:
+        return row[2]
+    return row[3]
+
+
 # ---------------------------------------------------------------------------
 # Window U-values  (Table 24)
 # ---------------------------------------------------------------------------
diff --git a/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py b/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py
index 86377466..d4af358c 100644
--- a/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py
+++ b/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py
@@ -28,6 +28,7 @@ from domain.ml.rdsap_uvalues import (
     WALL_TIMBER_FRAME,
     thermal_bridging_y,
     u_door,
+    u_exposed_floor,
     u_floor,
     u_party_wall,
     u_roof,
@@ -783,6 +784,22 @@ def test_u_floor_with_insulation_lowers_u_value() -> None:
     assert insulated < 0.3
 
 
+def test_u_exposed_floor_age_b_unknown_insulation_uses_table_20_row_a_to_g() -> None:
+    # Arrange — RdSAP10 §5.13 Table 20 (page 47) gives U-values for
+    # exposed and semi-exposed upper floors keyed on age band +
+    # insulation thickness. The "Insulation unknown or as built"
+    # column at age band A-G = 1.20 W/m²K. Elmhurst worksheet
+    # U985-0001-000490 Extension 1 records U=1.20 for its exposed
+    # timber floor (1900-1929, no insulation lodged) — this lookup
+    # reproduces that exact value without any geometry input.
+
+    # Act
+    result = u_exposed_floor(age_band="B", insulation_thickness_mm=None)
+
+    # Assert
+    assert result == pytest.approx(1.20, abs=0.001)
+
+
 def test_u_floor_falls_back_to_mid_range_when_geometry_unknown() -> None:
     # Arrange — geometry missing.