diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index c663e2fe..d56ffaaa 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -3835,7 +3835,16 @@ def _elmhurst_main_heating_category(
     membership is the authoritative signal. A PCDB-referenced boiler on
     mains/LPG gas is category 2 (gas-fired boilers). Other system types
     fall through to None so the cascade applies its default pumps_fans
-    130 kWh/yr until extended."""
+    130 kWh/yr until extended.
+
+    TODO: route Table 4a HP SAP codes (211-227, 521-527) to
+    category=4 when no PCDB Table 362 record is lodged. Currently
+    deferred because the correct dispatch needs (a) Table 12a
+    high/low rate split for HP-on-E7 cost cascade and (b) Table 4f
+    MEV / flue-fan / solar HW pump components for pumps_fans —
+    without both, naive category=4 dispatch overshoots cost by
+    £1.3k and undershoots pumps_fans by 252 kWh on cert 000565.
+    """
     if pcdb_index is not None and heat_pump_record(pcdb_index) is not None:
         return _ELMHURST_HEATING_CATEGORY_HEAT_PUMP
     if pcdb_index is not None and mh.fuel_type in _ELMHURST_GAS_BOILER_FUEL_TYPES:
diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index bd33cecc..c45932c5 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -757,7 +757,13 @@ def _space_heating_fuel_cost_gbp_per_kwh(
     """Space heating bills at the main fuel's rate. When the dwelling is
     on an off-peak tariff (meter_type != standard) AND the main fuel is
     electricity, bill at the off-peak rate instead. Trusts the cert's
-    meter_type rather than inferring tariff from heating code."""
+    meter_type rather than inferring tariff from heating code.
+
+    TODO: SAP 10.2 Table 12a applies a per-system high/low rate split
+    rather than the binary all-low / all-high implemented here. For
+    HP carriers on E7 the split is ~33% high / 67% low (cert 000565
+    empirically implies that split); single-rate biases the cost
+    £±1k vs the worksheet. Table 12a needs its own cascade slice."""
     if _is_electric_main(main) and _is_off_peak_meter(meter_type, fuel_is_electric=True):
         return prices.e7_low_rate_p_per_kwh * _PENCE_TO_GBP
     return _fuel_cost_gbp_per_kwh(main, prices)