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 762f7bb40..578cb67de 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -9161,6 +9161,52 @@ def test_water_only_heat_network_half_charge_adds_to_the_dwellings_fuel_standing
     assert abs(total - 84.0) <= 1e-9
 
 
+def test_water_only_code_on_a_heat_network_space_main_takes_the_full_charge_not_both() -> None:
+    # Arrange — SAP 10.2 §C6's "unless" clause. A water-only WHC (950) can sit
+    # on a cert whose SPACE main is ALSO a heat network (SAP 301). C6 gives the
+    # half "unless the space heating is also a heat network (see next
+    # paragraph)", and that next paragraph (p.55) says "the total standing
+    # charge is the normal heat network standing charge" — £120 flat.
+    #
+    # So the half must NOT stack onto the full charge here: £180 would be the
+    # double-count this branch exists to prevent, and £120 is not additive with
+    # the dwelling's fuel standing either ("the TOTAL standing charge IS").
+    from dataclasses import replace
+
+    from domain.sap10_calculator.tables.table_12a import Tariff
+
+    base = _typical_semi_detached_epc()
+    network_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=20,  # EPC mains gas (community)
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2306,
+        main_heating_category=6,
+        sap_main_heating_code=301,  # community boilers — a heat network
+    )
+    epc = replace(
+        base,
+        sap_heating=make_sap_heating(
+            main_heating_details=[network_main],
+            water_heating_code=950,
+            water_heating_fuel=20,
+        ),
+    )
+
+    # Act
+    total = _standing_charges_total_gbp(
+        epc,
+        network_main,
+        main_fuel_code=20,
+        water_heating_fuel_code=20,
+        tariff=Tariff.STANDARD,
+    )
+
+    # Assert — the full £120, not £120 + £60.
+    assert abs(total - 120.0) <= 1e-9
+
+
 def test_water_only_heat_network_half_charge_reaches_the_fuel_cost_worksheet() -> None:
     # Arrange — the §C6 half charge must reach worksheet (251), not just the
     # helper. A gas-boiler main (SAP 102, not a network) with WHC 950 keeps the