diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
index 1dafc14d..0c6b1e5e 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
@@ -215,3 +215,8 @@ LINE_63B_M_PV_DIVERTER_KWH: tuple[float, ...] = (0.0,) * 12
 LINE_63C_M_SOLAR_KWH: tuple[float, ...] = (0.0,) * 12
 LINE_63D_M_FGHRS_KWH: tuple[float, ...] = (0.0,) * 12
 LINE_64_M_OUTPUT_FROM_WH_KWH: tuple[float, ...] = LINE_62_M_TOTAL_WH_KWH
+LINE_64A_M_ELECTRIC_SHOWER_KWH: tuple[float, ...] = (0.0,) * 12
+LINE_65_M_HEAT_GAINS_FROM_WH_KWH: tuple[float, ...] = (
+    65.1690, 57.4789, 60.7300, 52.6612, 50.5342, 44.9825,
+    44.0196, 46.0942, 46.9564, 53.0172, 57.1669, 64.3662,
+)
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
index 0956a0c6..98a96d96 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
@@ -201,3 +201,8 @@ LINE_63B_M_PV_DIVERTER_KWH: tuple[float, ...] = (0.0,) * 12   # no PV diverter
 LINE_63C_M_SOLAR_KWH: tuple[float, ...] = (0.0,) * 12         # no solar HW
 LINE_63D_M_FGHRS_KWH: tuple[float, ...] = (0.0,) * 12         # no FGHRS
 LINE_64_M_OUTPUT_FROM_WH_KWH: tuple[float, ...] = LINE_62_M_TOTAL_WH_KWH  # no reductions
+LINE_64A_M_ELECTRIC_SHOWER_KWH: tuple[float, ...] = (0.0,) * 12  # mixer showers only
+LINE_65_M_HEAT_GAINS_FROM_WH_KWH: tuple[float, ...] = (
+    75.2034, 66.4381, 70.4305, 61.5896, 59.4711, 53.3391,
+    52.4705, 54.6991, 55.4582, 62.1383, 66.4342, 74.3403,
+)
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_water_heating.py b/packages/domain/src/domain/sap/worksheet/tests/test_water_heating.py
index db8b87d4..0c305ba6 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_water_heating.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_water_heating.py
@@ -23,6 +23,7 @@ from domain.sap.worksheet.water_heating import (
     combi_loss_monthly_kwh_table_3a_keep_hot_time_clock,
     distribution_loss_monthly_kwh,
     energy_content_of_hot_water_monthly_kwh,
+    heat_gains_from_water_heating_monthly_kwh,
     hot_water_baths_monthly_l_per_day,
     hot_water_mixer_showers_monthly_l_per_day,
     hot_water_other_uses_monthly_l_per_day,
@@ -554,6 +555,35 @@ def test_output_from_water_heater_clamps_to_zero_when_renewables_exceed_demand()
     assert all(v == pytest.approx(0.0, abs=1e-9) for v in monthly)
 
 
+@pytest.mark.parametrize("fixture", (_w000474, _w000490), ids=("000474", "000490"))
+def test_heat_gains_from_water_heating_matches_elmhurst_line_65(fixture) -> None:  # type: ignore[no-untyped-def]
+    """SAP10.2 §4 line (65)m heat gains released into the heated space:
+        (65)m = 0.25 × [0.85 × (45)m + (61)m + (64a)m]
+              + 0.8 × [(46)m + (57)m + (59)m]
+
+    The first bracket is delivered-heat losses (energy hot from the tap +
+    combi cycling losses + electric-shower waste heat) at 25% recovery;
+    the second is pipe-side losses (distribution + solar storage +
+    primary circuit) at 80% recovery. Per spec footnote (57)m is only
+    included when the hot water store is inside the dwelling (heat
+    networks excluded) — both fixtures have (57)m=0 so the conditional
+    doesn't bite.
+    """
+    # Arrange / Act
+    monthly = heat_gains_from_water_heating_monthly_kwh(
+        energy_content_monthly_kwh=fixture.LINE_45_M_HW_ENERGY_CONTENT_KWH,
+        distribution_loss_monthly_kwh=fixture.LINE_46_M_DISTRIBUTION_LOSS_KWH,
+        solar_storage_monthly_kwh=fixture.LINE_57_M_SOLAR_STORAGE_KWH,
+        primary_loss_monthly_kwh=fixture.LINE_59_M_PRIMARY_LOSS_KWH,
+        combi_loss_monthly_kwh=fixture.LINE_61_M_COMBI_LOSS_KWH,
+        electric_shower_monthly_kwh=fixture.LINE_64A_M_ELECTRIC_SHOWER_KWH,
+    )
+
+    # Assert
+    for m, (actual, exp) in enumerate(zip(monthly, fixture.LINE_65_M_HEAT_GAINS_FROM_WH_KWH)):
+        assert actual == pytest.approx(exp, abs=1e-3), f"month {m+1}"
+
+
 def test_assumed_occupancy_floor_at_n_eq_1_for_small_dwellings() -> None:
     """Appendix J piecewise definition: TFA ≤ 13.9 m² → N=1 exactly. A
     tiny studio flat at the boundary is the most common trigger."""
diff --git a/packages/domain/src/domain/sap/worksheet/water_heating.py b/packages/domain/src/domain/sap/worksheet/water_heating.py
index 9a20560d..039f6af9 100644
--- a/packages/domain/src/domain/sap/worksheet/water_heating.py
+++ b/packages/domain/src/domain/sap/worksheet/water_heating.py
@@ -302,6 +302,42 @@ def output_from_water_heater_monthly_kwh(
     )
 
 
+def heat_gains_from_water_heating_monthly_kwh(
+    *,
+    energy_content_monthly_kwh: tuple[float, ...],
+    distribution_loss_monthly_kwh: tuple[float, ...],
+    solar_storage_monthly_kwh: tuple[float, ...],
+    primary_loss_monthly_kwh: tuple[float, ...],
+    combi_loss_monthly_kwh: tuple[float, ...],
+    electric_shower_monthly_kwh: tuple[float, ...],
+) -> tuple[float, ...]:
+    """SAP 10.2 §4 line (65)m heat gains released into the heated space:
+        (65)m = 0.25 × [0.85 × (45)m + (61)m + (64a)m]
+              + 0.80 × [(46)m + (57)m + (59)m]
+
+    First bracket: delivered-heat losses (hot water at the tap + combi
+    cycling losses + electric-shower waste heat) at 25 % recovery into
+    the dwelling. Second bracket: pipe-side losses (distribution +
+    solar storage + primary circuit) at 80 % recovery — these run
+    through the heated envelope so most of the loss heats it.
+
+    Per spec footnote at xlsx row 302, include (57)m only if the hot
+    water store is in the dwelling. Callers should pass zero for
+    out-of-dwelling stores (e.g. communal heat networks).
+    """
+    return tuple(
+        0.25 * (0.85 * e + c + es) + 0.80 * (d + s + p)
+        for e, d, s, p, c, es in zip(
+            energy_content_monthly_kwh,
+            distribution_loss_monthly_kwh,
+            solar_storage_monthly_kwh,
+            primary_loss_monthly_kwh,
+            combi_loss_monthly_kwh,
+            electric_shower_monthly_kwh,
+        )
+    )
+
+
 def _days_weighted_average(monthly: tuple[float, ...]) -> float:
     """Σ value[m] × n_m / 365 — used by Appendix J equations J4 and J9."""
     return sum(v * d for v, d in zip(monthly, _DAYS_IN_MONTH)) / _DAYS_IN_YEAR