diff --git a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
index 5b0b63d7..8683e75c 100644
--- a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
@@ -185,6 +185,51 @@ _FORCE_SECONDARY_FOR_MAIN_CODES: Final[frozenset[int]] = frozenset(
 _PV_EXPORT_TARIFF_CODE: Final[int] = 60
 
 
+# SAP 10.2 Table 12c (page 193) — Distribution Loss Factor for heat
+# networks by dwelling age band, used when no PCDB record is available
+# (the modal RdSAP case). Per §C3.1: "Where a heat network is listed
+# in the PCDB, the DLF is already factored into the cost, CO2 and PE
+# factors recorded therein, so a DLF of 1 should be entered in
+# worksheet (306) to avoid double counting." For non-PCDB networks
+# (our case), DLF must be applied. K-or-newer (post-2007) = 1.50.
+_HEAT_NETWORK_DLF_BY_AGE: Final[dict[str, float]] = {
+    "A": 1.20, "B": 1.26, "C": 1.33, "D": 1.37, "E": 1.41, "F": 1.43,
+    "G": 1.45, "H": 1.46, "I": 1.48, "J": 1.49, "K": 1.50, "L": 1.50,
+    "M": 1.50,
+}
+_HEAT_NETWORK_DLF_DEFAULT: Final[float] = 1.50
+
+
+# SAP 10.2 Table 4a codes for heat-network main heating systems:
+#   301 = boiler-driven community heating
+#   302 = boiler-driven community heating with CHP
+#   303 = community CHP only
+#   304 = electric heat-pump community heating
+_HEAT_NETWORK_MAIN_CODES: Final[frozenset[int]] = frozenset({301, 302, 303, 304})
+_HEAT_NETWORK_CATEGORY: Final[int] = 6
+
+
+def _is_heat_network_main(main: Optional[MainHeatingDetail]) -> bool:
+    """True when the cert's main heating is a heat network — either by
+    SAP code (Table 4a 301-304) or by `main_heating_category` (6)."""
+    if main is None:
+        return False
+    code = main.sap_main_heating_code
+    if isinstance(code, int) and code in _HEAT_NETWORK_MAIN_CODES:
+        return True
+    return main.main_heating_category == _HEAT_NETWORK_CATEGORY
+
+
+def _heat_network_dlf(age_band: Optional[str]) -> float:
+    """RdSAP 10 §10.11 + SAP 10.2 Table 12c distribution loss factor by
+    age band. Defaults to the K-or-newer value (1.50) when band missing."""
+    if age_band is None:
+        return _HEAT_NETWORK_DLF_DEFAULT
+    return _HEAT_NETWORK_DLF_BY_AGE.get(
+        age_band.upper(), _HEAT_NETWORK_DLF_DEFAULT
+    )
+
+
 @dataclass(frozen=True)
 class PriceTable:
     """Seam between the spec-correct SAP 10.2/10.3 Table 12 prices and
@@ -749,12 +794,29 @@ def cert_to_inputs(
     )
 
     eff = seasonal_efficiency(main_code, main_category, main_fuel)
+    if _is_heat_network_main(main):
+        # SAP 10.2 Table 12 note (k): heat-network unit prices are per
+        # kWh of heat GENERATED (before distribution losses), not per
+        # kWh of fuel consumed. Setting efficiency = 1/DLF makes the
+        # calculator's `main_fuel_kwh = q_useful / (1/DLF) = q_useful
+        # × DLF = q_generated`, so cost = q_generated × unit_price as
+        # the spec requires.
+        eff = 1.0 / _heat_network_dlf(primary_age)
     water_eff = _water_efficiency_with_category_inherit(
         water_heating_code=epc.sap_heating.water_heating_code,
         main_code=main_code,
         main_category=main_category,
         main_fuel=main_fuel,
     )
+    if (
+        _is_heat_network_main(main)
+        and epc.sap_heating.water_heating_code in _WATER_INHERIT_FROM_MAIN_CODES
+    ):
+        # HW from main on a heat-network cert: the DHW also incurs the
+        # network's distribution losses. Same 1/DLF override as for
+        # space heating so the delivered HW kWh reflects q_useful × DLF
+        # = q_generated, matching the per-kWh-generated unit price.
+        water_eff = 1.0 / _heat_network_dlf(primary_age)
     is_instantaneous = epc.sap_heating.water_heating_code in _INSTANTANEOUS_WATER_CODES
     hw_kwh = predicted_hot_water_kwh(
         total_floor_area_m2=epc.total_floor_area_m2,
diff --git a/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
index 2d13e2f5..5798be33 100644
--- a/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
@@ -80,6 +80,133 @@ def _typical_semi_detached_epc():
     )
 
 
+def test_heat_network_main_applies_table12c_dlf_to_main_heating_efficiency() -> None:
+    # Arrange — heat-network main heating (Table 4a code 301 = community
+    # heating with CHP/boilers; main_heating_category=6). Cert age band
+    # E (1967-1975) lodges Table 12c DLF = 1.41.
+    # Per SAP 10.2 §C3.1 + Table 12 note (k): unit price is per kWh of
+    # heat GENERATED (i.e. before distribution losses), so the fuel-kwh
+    # multiplied by the unit price must be q_generated = q_useful × DLF.
+    # Setting main_heating_efficiency = 1/DLF makes our calculator's
+    # main_fuel_kwh = q_useful × DLF, which gives the correct cost.
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=20,  # mains gas (community)
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=6,
+        sap_main_heating_code=301,
+    )
+    part = make_building_part(construction_age_band="E")
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[part],
+        sap_heating=make_sap_heating(main_heating_details=[main]),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+
+    # Assert — DLF = 1.41 for age E → effective efficiency = 1/1.41 = 0.709.
+    assert inputs.main_heating_efficiency == pytest.approx(1.0 / 1.41, abs=0.005)
+
+
+def test_heat_network_main_with_hw_from_main_dlf_scales_hot_water_kwh() -> None:
+    # Arrange — when main heating is a heat network AND water heating
+    # inherits from main (water_heating_code=901), the HW also incurs
+    # the network's distribution losses. The water efficiency must be
+    # overridden to 1/DLF so that the delivered HW kWh (and therefore
+    # cost/CO2/PE applied to it) reflects q_useful × DLF.
+    # Compare against a gas-boiler baseline at the same age band: the
+    # heat-network HW kWh should be greater by the ratio 0.80/(1/DLF) =
+    # DLF × 0.80 = 0.80 × 1.41 = 1.128 (i.e. ~13% higher) since the
+    # non-heat-network baseline inherits water efficiency 0.80 from
+    # the heat-network main's pre-DLF efficiency.
+    part = make_building_part(construction_age_band="E")
+
+    hn_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=20,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=6,
+        sap_main_heating_code=301,
+    )
+    hn_epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[part],
+        sap_heating=make_sap_heating(
+            main_heating_details=[hn_main],
+            water_heating_code=901,  # from main
+        ),
+    )
+
+    # Comparable gas-boiler baseline that ALSO inherits a 0.80 water
+    # efficiency through `water_heating_code=901` for direct comparison.
+    # Use sap_main_heating_code = None so cascade returns 0.80 default.
+    gas_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=26,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=2,
+    )
+    gas_epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[part],
+        sap_heating=make_sap_heating(
+            main_heating_details=[gas_main],
+            water_heating_code=901,
+        ),
+    )
+
+    # Act
+    hn_hw = cert_to_inputs(hn_epc).hot_water_kwh_per_yr
+    gas_hw = cert_to_inputs(gas_epc).hot_water_kwh_per_yr
+
+    # Assert — DLF (1.41) for age E × 0.80 baseline / (1/1.41) HN = 1.128.
+    assert hn_hw / gas_hw == pytest.approx(1.41 * 0.80 / 1.0, abs=0.02)
+
+
+def test_gas_boiler_main_efficiency_unchanged_by_dlf_override() -> None:
+    # Arrange — regression check: the DLF override only fires for heat-
+    # network main heating. A standard gas boiler (cat=2, code=102) must
+    # still get its Table 4b winter efficiency (0.84).
+
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=26,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=2,
+        sap_main_heating_code=102,  # gas combi pre-2005, 0.84 eff
+    )
+    part = make_building_part(construction_age_band="E")
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[part],
+        sap_heating=make_sap_heating(main_heating_details=[main]),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+
+    # Assert — Table 4b code 102 winter efficiency = 0.84, no DLF override.
+    assert inputs.main_heating_efficiency == pytest.approx(0.84, abs=0.001)
+
+
 def test_main_heating_fraction_does_not_override_table11_secondary_default() -> None:
     # Arrange — the S-B30 attempt assumed `main_heating_fraction=1` meant
     # "no secondary heating" and dropped the Table 11 default in that