diff --git a/domain/sap10_calculator/tables/table_12a.py b/domain/sap10_calculator/tables/table_12a.py
index fe04aaaa..dccc555e 100644
--- a/domain/sap10_calculator/tables/table_12a.py
+++ b/domain/sap10_calculator/tables/table_12a.py
@@ -16,7 +16,7 @@ all consumption at the unit price.
 from __future__ import annotations
 
 from enum import Enum
-from typing import Final
+from typing import Final, Optional
 
 
 class Table12aSystem(Enum):
@@ -191,7 +191,14 @@ def other_use_high_rate_fraction(use: OtherUse, tariff: Tariff) -> float:
 def tariff_from_meter_type(meter_type: object) -> Tariff:
     """Resolve the RdSAP cert `meter_type` field to a Table 12a tariff
     column. Unknown / missing → STANDARD (no off-peak split applied)
-    per the Q11b spec-faithful policy."""
+    per the Q11b spec-faithful policy.
+
+    NOTE: for a Dual meter the §12 dispatch (Rules 1-4 page 62)
+    requires the main heating SAP codes to choose between 7-hour and
+    10-hour. This helper returns the SEVEN_HOUR default for Dual —
+    callers that have access to the main heating codes should use
+    `rdsap_tariff_for_cert` instead.
+    """
     if meter_type is None:
         return Tariff.STANDARD
     if isinstance(meter_type, int):
@@ -202,3 +209,85 @@ def tariff_from_meter_type(meter_type: object) -> Tariff:
             return Tariff.STANDARD
         return _METER_INT_TO_TARIFF[code]
     return Tariff.STANDARD
+
+
+# RdSAP 10 §12 page 62 — SAP main heating code sets for the Dual-meter
+# tariff dispatch. Each rule's set is taken verbatim from §12 Rules 1-3.
+# Rule 1: Electric CPSU → 10-hour
+_RULE_1_CPSU_CODES: Final[frozenset[int]] = frozenset({192})
+# Rule 2: storage-based electric → 7-hour. The (421, 422) underfloor
+# subset is explicit per §12 ("421 or 422, but not 424") — 423 / 425
+# fall through to Rule 4 default unless a later spec amendment adds
+# them.
+_RULE_2_STORAGE_CODES: Final[frozenset[int]] = frozenset(
+    list(range(401, 410))   # electric storage heaters 401-409
+    + [193, 195]            # electric dry-core / water-storage boiler
+    + [421, 422]            # electric underfloor heating (424 excluded)
+)
+# Rule 3: direct-acting electric + heat pumps + electric room heaters
+# → 10-hour. §12 lists "heat pump (211 to 224, 521 to 524, or
+# database)" — the "database" branch fires when the cert lodges a
+# PCDB Table 362 heat-pump index regardless of SAP code.
+_RULE_3_TEN_HOUR_CODES: Final[frozenset[int]] = frozenset(
+    [191]                       # direct-acting electric boiler
+    + list(range(211, 225))     # heat pumps 211-224
+    + list(range(521, 525))     # warm-air heat pumps 521-524
+    # TODO: electric room heater codes (SAP Table 4a row 6xx for
+    # electric panel / radiant heaters) when a fixture surfaces them.
+)
+
+
+def rdsap_tariff_for_cert(
+    meter_type: object,
+    *,
+    main_1_sap_code: Optional[int] = None,
+    main_2_sap_code: Optional[int] = None,
+    main_1_is_heat_pump_database: bool = False,
+    main_2_is_heat_pump_database: bool = False,
+) -> Tariff:
+    """RdSAP 10 §12 page 62 — full meter+heating tariff dispatch.
+
+    Single meter → STANDARD. Dual 18-hour / Dual 24-hour map straight
+    to their respective tariffs. Otherwise applies §12 Rules 1-4
+    where each rule considers BOTH main heating systems on multi-
+    main certs ("the main system or either main system if there are
+    two"):
+
+      Rule 1  Electric CPSU (192)                    → 10-hour
+      Rule 2  Storage / storage boiler / underfloor  → 7-hour
+              (401-409, 193, 195, 421, 422)
+      Rule 3  Direct-acting electric boiler (191),   → 10-hour
+              heat pump (211-224, 521-524, database),
+              electric room heaters
+      Rule 4  None of the above                      → 7-hour
+              (default for Dual + non-electric main)
+
+    `main_1_is_heat_pump_database` / `main_2_is_heat_pump_database`
+    signal the "or database" Rule 3 branch — the cert lodges a PCDB
+    Table 362 heat-pump record. Callers compute this via
+    `heat_pump_record(main_heating_index_number) is not None`.
+
+    Cert 000565 (Main 1 SAP code 224 ASHP + Dual meter) → Rule 3 →
+    TEN_HOUR, matching the worksheet's "10 Hour Off Peak" lodging.
+    """
+    base = tariff_from_meter_type(meter_type)
+    # Non-Dual meters resolve straight from the meter type.
+    if base is not Tariff.SEVEN_HOUR:
+        return base
+    main_codes = {
+        c for c in (main_1_sap_code, main_2_sap_code) if c is not None
+    }
+    # Rule 1
+    if main_codes & _RULE_1_CPSU_CODES:
+        return Tariff.TEN_HOUR
+    # Rule 2 — checked BEFORE rule 3 per §12 ordering (storage takes
+    # precedence over the broader Rule 3 electric set).
+    if main_codes & _RULE_2_STORAGE_CODES:
+        return Tariff.SEVEN_HOUR
+    # Rule 3
+    if main_codes & _RULE_3_TEN_HOUR_CODES:
+        return Tariff.TEN_HOUR
+    if main_1_is_heat_pump_database or main_2_is_heat_pump_database:
+        return Tariff.TEN_HOUR
+    # Rule 4 — default
+    return Tariff.SEVEN_HOUR