diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index e387c73c9..109dabaed 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -1079,6 +1079,68 @@ def test_electric_boilers_191_195_map_to_distinct_table_12a_grid1_rows() -> None
     assert space_heating_high_rate_fraction(storage, Tariff.SEVEN_HOUR) == 0.00
 
 
+def test_electric_underfloor_codes_map_to_their_table_12a_grid1_rows() -> None:
+    # Arrange — SAP 10.2 Table 12a Grid 1 (PDF p.184/191) names the underfloor
+    # codes explicitly: "Integrated (storage+direct-acting) systems (applies to
+    # storage heaters 408 and underfloor heating 422 and 423)" → 7-hour 0.20;
+    # "Underfloor heating (in screed above insulation, in timber floor or
+    # immediately below floor covering)" (424/425) → 7-hour 0.90 / 10-hour
+    # 0.50. Code 421 ("in concrete slab, off-peak only") is named in NEITHER
+    # underfloor row — the slab is the storage medium, so it takes "Other
+    # storage heaters" (0.00 at 7-hour or 24-hour), consistent with §12 Rule 2
+    # listing 421 among the off-peak-implying storage codes. Before this
+    # resolver branch every underfloor code fell to the None fallback and
+    # billed space heat 100% at the night rate — on portfolio 796, 22 of the
+    # 90 underfloor-override dwellings sit on a 7-hour Dual meter and were
+    # over-credited (e.g. screed 424 at 5.50p/kWh instead of the blended
+    # ~14.3p).
+    from domain.sap10_calculator.tables.table_12a import (
+        Table12aSystem,
+        Tariff,
+        space_heating_high_rate_fraction,
+    )
+
+    def _underfloor_main(code: int) -> MainHeatingDetail:
+        return MainHeatingDetail(
+            has_fghrs=False,
+            main_fuel_type=29,  # electricity
+            heat_emitter_type=0,
+            emitter_temperature=1,
+            main_heating_control=2701,
+            main_heating_category=8,  # electric underfloor
+            sap_main_heating_code=code,
+        )
+
+    # Act
+    slab = _table_12a_system_for_main(_underfloor_main(421))
+    integrated = [_table_12a_system_for_main(_underfloor_main(c)) for c in (422, 423)]
+    screed = [_table_12a_system_for_main(_underfloor_main(c)) for c in (424, 425)]
+
+    # Assert — the three spec rows, with their published fractions.
+    assert slab is Table12aSystem.OTHER_STORAGE_HEATERS
+    assert all(s is Table12aSystem.INTEGRATED_STORAGE_DIRECT for s in integrated)
+    assert all(s is Table12aSystem.UNDERFLOOR_HEATING for s in screed)
+    assert space_heating_high_rate_fraction(slab, Tariff.SEVEN_HOUR) == 0.00
+    assert (
+        space_heating_high_rate_fraction(
+            Table12aSystem.INTEGRATED_STORAGE_DIRECT, Tariff.SEVEN_HOUR
+        )
+        == 0.20
+    )
+    assert (
+        space_heating_high_rate_fraction(
+            Table12aSystem.UNDERFLOOR_HEATING, Tariff.SEVEN_HOUR
+        )
+        == 0.90
+    )
+    assert (
+        space_heating_high_rate_fraction(
+            Table12aSystem.UNDERFLOOR_HEATING, Tariff.TEN_HOUR
+        )
+        == 0.50
+    )
+
+
 def test_storage_heater_classified_by_sap_code_over_stale_category() -> None:
     # Regression: electric STORAGE heaters must resolve to a storage Table 12a
     # Grid 1 row from their Table 4a SAP code (401-409) even when the RdSAP