diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index c8a59fcf..a271f8f5 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -7216,7 +7216,21 @@ def cert_to_inputs(
         epc=epc,
     )
     if apm_efficiencies is not None:
-        eff, water_eff = apm_efficiencies
+        # η_space (N3.6) always replaces the Table 4a default — the heat
+        # pump is the space main. η_water (N3.7a) applies ONLY when the DHW
+        # is actually heated by that main (WHC "from main": 901/902/914). A
+        # separate electric immersion (WHC 903) or other independent DHW
+        # source keeps its own water efficiency (immersion = 100%), not the
+        # HP's water SCOP — otherwise a HP-space + immersion-DHW dwelling
+        # under-counts its hot-water fuel (case 45: water 2130 -> 1894 kWh,
+        # +1.5 SAP, because 187.5% × 0.6 in-use = 112.5% was applied where
+        # the worksheet (216) uses 100%).
+        eff, apm_water_eff = apm_efficiencies
+        if (
+            epc.sap_heating.water_heating_code
+            in _WATER_INHERIT_FROM_MAIN_CODES
+        ):
+            water_eff = apm_water_eff
     if (
         _is_heat_network_main(main)
         and epc.sap_heating.water_heating_code in _WATER_INHERIT_FROM_MAIN_CODES
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 4531c08d..e3e99e9e 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -38,6 +38,7 @@ from datatypes.epc.domain.epc_property_data import (
 from domain.sap10_ml.tests._fixtures import (
     make_building_part,
     make_floor_dimension,
+    make_main_heating_detail,
     make_minimal_sap10_epc,
     make_sap_heating,
     make_window,
@@ -7581,3 +7582,55 @@ def test_index_less_mev_applies_table_4g_note_3_default_data_iuf() -> None:
     # 2.5x the raw-0.8 value, not the raw default.
     assert fan_kwh > 0.0
     assert abs(fan_kwh - expected) <= 1e-9
+
+
+def test_heat_pump_water_scop_not_applied_to_separate_immersion_dhw() -> None:
+    # Arrange — SAP 10.2 Appendix N3.7(a): a heat pump's PCDB water
+    # efficiency (η_water) applies to the DHW ONLY when the cylinder is
+    # heated BY the heat pump. A separate electric immersion (WHC 903) heats
+    # the water at 100% regardless of the space-heating system, so the HP's
+    # water SCOP must NOT leak onto it. Invariant: a WHC-903 immersion's
+    # hot-water fuel is INDEPENDENT of the main — a heat-pump main and a gas-
+    # boiler main yield the SAME immersion fuel (both 100%, no primary loss).
+    # Before the fix the APM override set η_water = 187.5% × 0.6 in-use =
+    # 112.5% on the HP cert, under-counting its immersion fuel. Worksheet-
+    # validated on simulated case 45: water (62) = 2130.26 kWh at η_water=100%,
+    # not 2130.26 / 1.125 = 1893.57.
+    hp_main = make_main_heating_detail(
+        main_fuel_type=29,  # electricity
+        heat_emitter_type=1,
+        main_heating_category=4,  # heat pump
+        main_heating_index_number=100053,  # PCDB Table 362 ASHP (ECODAN 5 kW)
+        main_heating_data_source=1,
+    )
+    boiler_main = make_main_heating_detail(
+        main_fuel_type=26,  # mains gas
+        heat_emitter_type=1,
+        main_heating_category=2,  # gas boiler
+        sap_main_heating_code=102,
+    )
+
+    def _immersion_epc(main: MainHeatingDetail) -> EpcPropertyData:
+        return make_minimal_sap10_epc(
+            total_floor_area_m2=_TYPICAL_TFA_M2,
+            habitable_rooms_count=4,
+            country_code="ENG",
+            has_hot_water_cylinder=True,
+            sap_heating=make_sap_heating(
+                main_heating_details=[main],
+                water_heating_code=903,  # separate electric immersion
+                water_heating_fuel=30,  # standard electricity
+                cylinder_size=2,
+                cylinder_insulation_type=1,
+                cylinder_insulation_thickness_mm=25,
+            ),
+        )
+
+    # Act
+    hp_fuel = cert_to_inputs(_immersion_epc(hp_main)).hot_water_kwh_per_yr
+    boiler_fuel = cert_to_inputs(_immersion_epc(boiler_main)).hot_water_kwh_per_yr
+
+    # Assert — the immersion DHW fuel is identical whether the space main is a
+    # heat pump or a gas boiler (the HP water SCOP does not apply to it).
+    assert hp_fuel > 0.0
+    assert abs(hp_fuel - boiler_fuel) <= 1e-6