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 1ac7e55f..571d27ab 100644
--- a/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py
+++ b/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py
@@ -1283,3 +1283,57 @@ def test_u_roof_matches_table16_for_every_cohort_description_thickness_pair(
 
     # Assert
     assert abs(u - expected_u) < 1e-4
+
+
+# ----- §5.12 formula cascade cohort pins (Floors) -----
+#
+# u_floor is formula-driven (BS EN ISO 13370 + RdSAP10 §5.12) rather
+# than table-lookup, so each pin asserts a per-geometry value derived
+# by hand from the spec formula. Two cases from cert 0240 (main +
+# extension) cover the dt < B and dt > B branches of the solid-floor
+# branch; suspended-floor + Table 19 footnote (2) overrides land in
+# follow-on slices when cohort coverage demands them.
+#
+# Hand-derivation for the first row (cert 0240 bp0):
+#   age J → Table 19 default insulation = 75 mm
+#   w = 0.3 m (default), g = 1.5, Rsi+Rse = 0.21, Rf = 0.001×75/0.035 = 2.143
+#   dt = 0.3 + 1.5×(0.21 + 2.143) = 3.829
+#   B  = 2×97.72/36.45 = 5.362  → dt < B branch
+#   U  = 2g·ln(πB/dt + 1)/(πB + dt) = 0.2447 → rounds to 0.24
+_FLOOR_FORMULA_COHORT_PINS: tuple[tuple[str, str, Optional[int], float, float, Optional[int], float], ...] = (
+    # (description, age, construction, area_m2, perimeter_m, wall_thickness_mm, expected_u)
+    ("Solid, insulated (assumed)", "J", 1, 97.72, 36.45, None, 0.24),  # cert 0240 bp0 (dt < B)
+    ("Solid, insulated (assumed)", "J", 1, 20.61, 13.45, None, 0.29),  # cert 0240 bp1 (dt > B)
+)
+
+
+@pytest.mark.parametrize(
+    "description, age_band, construction, area_m2, perimeter_m, wall_thickness_mm, expected_u",
+    _FLOOR_FORMULA_COHORT_PINS,
+)
+def test_u_floor_matches_section_5_12_formula_for_cohort_geometry(
+    description: str,
+    age_band: str,
+    construction: Optional[int],
+    area_m2: float,
+    perimeter_m: float,
+    wall_thickness_mm: Optional[int],
+    expected_u: float,
+) -> None:
+    # Arrange — inputs replicate what `heat_transmission_from_cert` feeds
+    # `u_floor` for the corresponding building part in the cohort.
+
+    # Act
+    u = u_floor(
+        country=Country.ENG,
+        age_band=age_band,
+        construction=construction,
+        insulation_thickness_mm=None,
+        area_m2=area_m2,
+        perimeter_m=perimeter_m,
+        wall_thickness_mm=wall_thickness_mm,
+        description=description,
+    )
+
+    # Assert
+    assert abs(u - expected_u) < 1e-4