diff --git a/domain/sap10_calculator/tables/table_12a.py b/domain/sap10_calculator/tables/table_12a.py
index d79d9fbe..72ea8f7b 100644
--- a/domain/sap10_calculator/tables/table_12a.py
+++ b/domain/sap10_calculator/tables/table_12a.py
@@ -262,6 +262,19 @@ _RULE_3_TEN_HOUR_CODES: Final[frozenset[int]] = frozenset(
     + [691, 692, 693, 694, 699]  # electric room heaters (Table 4a)
 )
 
+# §12 Unknown-meter exception: "main heating is ground source or water source
+# heat pump" makes the dwelling dual even though the same heat pump on a Single
+# meter stays standard. This is the GROUND/WATER-source subset of Table 4a
+# (SAP 10.2 PDF p.176-177): ground 211/215/221/225 + warm-air 521/525, water
+# 213/216/223/226 + warm-air 523/526. AIR-source (214/217/224/227, 524/527) is
+# DELIBERATELY EXCLUDED — the spec names only ground/water source. A heat pump
+# lodged via a PCDB database index without one of these SAP codes can't have
+# its source type read from the code alone (coverage gap: rare, 0 corpus certs).
+_GROUND_OR_WATER_SOURCE_HEAT_PUMP_CODES: Final[frozenset[int]] = frozenset(
+    {211, 215, 221, 225, 521, 525}   # ground source (radiator + warm-air)
+    | {213, 216, 223, 226, 523, 526}  # water source (radiator + warm-air)
+)
+
 
 def _meter_is_unknown(meter_type: object) -> bool:
     """True when the meter is the RdSAP "Unknown" sentinel (code 3 / the
@@ -367,11 +380,18 @@ def rdsap_tariff_for_cert(
     # immersion); every other Unknown+dual-immersion cert already has a
     # storage main (Rule 2). Single-immersion 691 certs (Flat 7, Flat 2) and
     # whc-909 instantaneous certs correctly STAY standard — they carry no
-    # text-box off-peak system. (GSHP/WSHP-main trigger: separate follow-up.)
+    # text-box off-peak system. The §12 third exception bullet ("main heating
+    # is ground source or water source heat pump") is the
+    # `_GROUND_OR_WATER_SOURCE_HEAT_PUMP_CODES` check — AIR-source heat pumps
+    # (214/224 etc.) are NOT a trigger and stay standard (e.g. corpus certs
+    # "3/10 Bedford House", main 214 ASHP on Unknown meters — verified 2026-06-
+    # 23 they keep STANDARD). No corpus cert carries a GSHP/WSHP on an Unknown
+    # meter, so this trigger is a spec-completeness forward guard (0 impact).
     if _meter_is_unknown(meter_type):
         off_peak_evidence = (
             bool(main_codes & _RULE_1_CPSU_CODES)
             or bool(main_codes & _RULE_2_STORAGE_CODES)
+            or bool(main_codes & _GROUND_OR_WATER_SOURCE_HEAT_PUMP_CODES)
             or water_is_off_peak_dual_immersion
         )
         if off_peak_evidence:
diff --git a/tests/domain/sap10_calculator/test_table_12a.py b/tests/domain/sap10_calculator/test_table_12a.py
index 8ce42b09..9a70dbfa 100644
--- a/tests/domain/sap10_calculator/test_table_12a.py
+++ b/tests/domain/sap10_calculator/test_table_12a.py
@@ -106,6 +106,25 @@ def test_unknown_meter_dual_electric_immersion_triggers_off_peak_via_rules() ->
     ) is Tariff.STANDARD
 
 
+def test_unknown_meter_ground_or_water_source_heat_pump_triggers_off_peak() -> None:
+    # Arrange — RdSAP 10 §12 (PDF p.62) third Unknown-meter exception bullet:
+    # "main heating is ground source or water source heat pump." Per SAP 10.2
+    # Table 4a (PDF p.176-177) ground source = 211/215/221/225 (+ warm-air
+    # 521/525), water source = 213/216/223/226 (+ warm-air 523/526); AIR
+    # source (214/224 etc.) is NOT named, so it stays standard. Once the GSHP/
+    # WSHP makes the meter dual, Rules 1-4 (Rule 3, heat pump 211-224) → 10h.
+    # 0 corpus certs carry a GSHP/WSHP on an Unknown meter (the only Unknown-
+    # meter HPs are 3/10 Bedford House, main 214 = AIR source) — this is a
+    # spec-completeness forward guard.
+
+    # Act / Assert — ground/water source → 10-hour; air source → STANDARD.
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=211) is Tariff.TEN_HOUR
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=213) is Tariff.TEN_HOUR
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=521) is Tariff.TEN_HOUR
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=214) is Tariff.STANDARD
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=224) is Tariff.STANDARD
+
+
 def test_unknown_meter_with_non_electric_main_stays_standard() -> None:
     # Arrange — an "Unknown" meter on a GAS-heated dwelling (SAP 102) has
     # no off-peak evidence, so it must NOT pick up the Rule-4 Dual default