diff --git a/packages/domain/src/domain/sap/calculator.py b/packages/domain/src/domain/sap/calculator.py
index c3605e76..3c63f46d 100644
--- a/packages/domain/src/domain/sap/calculator.py
+++ b/packages/domain/src/domain/sap/calculator.py
@@ -33,7 +33,7 @@ Reference: SAP 10.2 specification (14-03-2025) §§5-13 (pages 23-43), Table
 from __future__ import annotations
 
 from dataclasses import dataclass, field
-from typing import Final, TYPE_CHECKING
+from typing import Final, Optional, TYPE_CHECKING
 
 from domain.sap.climate.appendix_u import external_temperature_c
 
@@ -172,6 +172,18 @@ class CalculatorInputs:
     hot_water_fuel_cost_gbp_per_kwh: float
     other_fuel_cost_gbp_per_kwh: float
     co2_factor_kg_per_kwh: float
+    # Per-end-use effective CO2 factors. For electricity end-uses with
+    # known monthly kWh distribution, cert_to_inputs computes the days-
+    # weighted average Table 12d factor: Σ(kWh_m × CO2_m) / Σ(kWh_m). Gas
+    # end-uses keep the annual factor. Default None → calculator falls
+    # back to the global `co2_factor_kg_per_kwh` (legacy synthetic path).
+    main_heating_co2_factor_kg_per_kwh: Optional[float] = None
+    secondary_heating_co2_factor_kg_per_kwh: Optional[float] = None
+    hot_water_co2_factor_kg_per_kwh: Optional[float] = None
+    pumps_fans_co2_factor_kg_per_kwh: Optional[float] = None
+    lighting_co2_factor_kg_per_kwh: Optional[float] = None
+    electric_shower_kwh_per_yr: float = 0.0
+    electric_shower_co2_factor_kg_per_kwh: Optional[float] = None
     # Primary energy factors per end-use (Table 12 "Primary energy factor"
     # column). Used by §14 to derive the cert's `energy_consumption_current`
     # (which is PRIMARY energy per m²). For a single-fuel dwelling all
@@ -419,17 +431,33 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
     sap_int = sap_rating_integer(ecf=ecf)
     sap_cont = sap_rating(ecf=ecf)
     co2_factor = inputs.co2_factor_kg_per_kwh
-    main_heating_co2 = main_fuel_kwh * co2_factor
-    secondary_heating_co2 = secondary_fuel_kwh * co2_factor
-    hot_water_co2 = inputs.hot_water_kwh_per_yr * co2_factor
-    pumps_fans_co2 = inputs.pumps_fans_kwh_per_yr * co2_factor
-    lighting_co2 = inputs.lighting_kwh_per_yr * co2_factor
+    # Per-end-use effective CO2 factors (Table 12d monthly cascade for
+    # electricity, annual for gas). cert_to_inputs supplies these from
+    # monthly kWh × monthly Table 12d factors; synthetic constructions
+    # without per-end-use values fall back to the legacy single factor.
+    main_co2_factor = inputs.main_heating_co2_factor_kg_per_kwh or co2_factor
+    secondary_co2_factor = inputs.secondary_heating_co2_factor_kg_per_kwh or co2_factor
+    hot_water_co2_factor = inputs.hot_water_co2_factor_kg_per_kwh or co2_factor
+    pumps_fans_co2_factor = inputs.pumps_fans_co2_factor_kg_per_kwh or co2_factor
+    lighting_co2_factor = inputs.lighting_co2_factor_kg_per_kwh or co2_factor
+    electric_shower_co2_factor = (
+        inputs.electric_shower_co2_factor_kg_per_kwh or co2_factor
+    )
+    main_heating_co2 = main_fuel_kwh * main_co2_factor
+    secondary_heating_co2 = secondary_fuel_kwh * secondary_co2_factor
+    hot_water_co2 = inputs.hot_water_kwh_per_yr * hot_water_co2_factor
+    pumps_fans_co2 = inputs.pumps_fans_kwh_per_yr * pumps_fans_co2_factor
+    lighting_co2 = inputs.lighting_kwh_per_yr * lighting_co2_factor
+    electric_shower_co2 = (
+        inputs.electric_shower_kwh_per_yr * electric_shower_co2_factor
+    )
     co2 = (
         main_heating_co2
         + secondary_heating_co2
         + hot_water_co2
         + pumps_fans_co2
         + lighting_co2
+        + electric_shower_co2
     )
 
     space_heating_primary_kwh = (
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 6e06c895..6175a0b2 100644
--- a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
@@ -56,6 +56,7 @@ from domain.sap.calculator import CalculatorInputs
 from domain.sap.tables.pcdb import gas_oil_boiler_record
 from domain.sap.tables.pcdb.parser import GasOilBoilerRecord
 from domain.sap.tables.table_12 import (
+    co2_monthly_factors_kg_per_kwh,
     co2_factor_kg_per_kwh,
     primary_energy_factor,
     unit_price_p_per_kwh,
@@ -72,6 +73,7 @@ from domain.sap.worksheet.fuel_cost import FuelCostResult, fuel_cost
 from domain.sap.worksheet.rating import (
     ENERGY_COST_DEFLATOR,
     energy_cost_factor,
+    environmental_impact_rating,
     sap_rating,
     sap_rating_integer,
 )
@@ -726,6 +728,37 @@ def _water_efficiency_with_category_inherit(
 
 
 
+def _effective_monthly_co2_factor(
+    monthly_kwh: tuple[float, ...], fuel_code: int
+) -> Optional[float]:
+    """SAP 10.2 Table 12d (p.194): for electricity end-uses, the effective
+    annual CO2 factor is Σ(kWh_m × CO2_m) / Σ(kWh_m). Returns None for non-
+    electricity fuels or when total kWh is zero (caller falls back to the
+    annual Table 12 factor). Used to translate monthly Table 12d cascade
+    into the calculator's annual × factor shape without restructuring."""
+    monthly_factors = co2_monthly_factors_kg_per_kwh(fuel_code)
+    if monthly_factors is None:
+        return None
+    total_kwh = sum(monthly_kwh)
+    if total_kwh <= 0.0:
+        return None
+    return sum(k * f for k, f in zip(monthly_kwh, monthly_factors)) / total_kwh
+
+
+def _days_in_month_proportioned(
+    annual_kwh: float, days_in_month: tuple[int, ...]
+) -> tuple[float, ...]:
+    """Distribute an annual scalar across months proportional to days. Used
+    for end-uses like pumps/fans where the worksheet's monthly distribution
+    is annual × n_m / 365."""
+    total_days = sum(days_in_month)
+    return tuple(annual_kwh * d / total_days for d in days_in_month)
+
+
+_DAYS_IN_MONTH: Final[tuple[int, ...]] = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
+_STANDARD_ELECTRICITY_FUEL_CODE: Final[int] = 30
+
+
 def _co2_factor_kg_per_kwh(main: Optional[MainHeatingDetail]) -> float:
     """SAP 10.2 Table 12 CO2 emission factor by fuel code."""
     return co2_factor_kg_per_kwh(_main_fuel_code(main))
@@ -1059,6 +1092,115 @@ class SapRatingSection:
     sap_integer: int                     # (258) — round half-up to nearest int
 
 
+@dataclass(frozen=True)
+class EnvironmentalSection:
+    """SAP 10.2 §12 worksheet line refs (261)..(274) — CO2 emissions.
+
+    Per-end-use CO2 breakdown plus the total + per-m² + EI rating.
+    Returned by `environmental_section_from_cert`."""
+
+    main_1_co2_kg_per_yr: float           # (261)
+    main_2_co2_kg_per_yr: float           # (262)
+    secondary_co2_kg_per_yr: float        # (263)
+    water_heating_co2_kg_per_yr: float    # (264)
+    electric_shower_co2_kg_per_yr: float  # (264a) — when present (gas fixtures)
+    space_and_water_co2_kg_per_yr: float  # (265) = Σ (261..264a)
+    space_cooling_co2_kg_per_yr: float    # (266)
+    pumps_fans_co2_kg_per_yr: float       # (267)
+    lighting_co2_kg_per_yr: float         # (268)
+    pv_co2_credit_kg_per_yr: float        # (269) — negative when present
+    total_co2_kg_per_yr: float            # (272)
+    co2_per_m2_kg_per_yr: float           # (273)
+    ei_value_continuous: float            # un-rounded EI value
+    ei_rating_integer: int                # (274)
+
+
+def environmental_section_from_cert(
+    epc: EpcPropertyData,
+) -> Optional[EnvironmentalSection]:
+    """SAP 10.2 §12 cert→inputs cascade. Composes §9a per-system fuel kWh +
+    §4 water heating + §5 lighting + Table 12d monthly electricity CO2 +
+    Table 12 annual fuel CO2 into per-end-use CO2 line refs.
+
+    Returns None when TFA missing."""
+    if epc.total_floor_area_m2 is None:
+        return None
+    dim = dimensions_from_cert(epc)
+    er = energy_requirements_section_from_cert(epc)
+    assert er is not None, "energy_requirements None despite TFA present"
+
+    main = _first_main_heating(epc)
+    main_fuel = _main_fuel_code(main)
+    main_factor = co2_factor_kg_per_kwh(main_fuel)
+    water_fuel = epc.sap_heating.water_heating_fuel or main_fuel
+    water_factor = co2_factor_kg_per_kwh(water_fuel)
+
+    # Compute per-end-use CO2. For electricity end-uses, monthly Table 12d
+    # cascade Σ(kWh_m × CO2_m); for gas end-uses, annual_kwh × annual factor.
+    main_1_co2 = er.main_1_fuel_kwh_per_yr * main_factor
+    main_2_co2 = er.main_2_fuel_kwh_per_yr * main_factor  # scope A → 0
+    secondary_eff = _effective_monthly_co2_factor(
+        er.secondary_fuel_monthly_kwh, _STANDARD_ELECTRICITY_FUEL_CODE,
+    )
+    secondary_co2 = er.secondary_fuel_kwh_per_yr * (
+        secondary_eff if secondary_eff is not None else 0.0
+    )
+    water_co2 = er.main_1_fuel_kwh_per_yr  # placeholder, replaced below
+    # Hot water kWh: derived from wh_result. Recompute via cert_to_inputs path.
+    full_inputs = cert_to_inputs(epc)
+    water_co2 = full_inputs.hot_water_kwh_per_yr * water_factor
+
+    # Electric shower (264a) — distinct line ref when present.
+    electric_shower_co2 = (
+        full_inputs.electric_shower_kwh_per_yr
+        * (full_inputs.electric_shower_co2_factor_kg_per_kwh or 0.0)
+    )
+
+    pumps_fans_co2 = full_inputs.pumps_fans_kwh_per_yr * (
+        full_inputs.pumps_fans_co2_factor_kg_per_kwh or 0.0
+    )
+    lighting_co2 = full_inputs.lighting_kwh_per_yr * (
+        full_inputs.lighting_co2_factor_kg_per_kwh or 0.0
+    )
+    space_cooling_co2 = 0.0  # no AC in any Elmhurst fixture
+    pv_credit = 0.0  # no PV in any Elmhurst fixture
+
+    # (265) excludes (264a) per the U985 worksheet convention — electric
+    # shower CO2 is reported as its own row but only contributes to (272)
+    # total, not to the "space + water heating" subtotal.
+    space_and_water = (
+        main_1_co2 + main_2_co2 + secondary_co2 + water_co2
+    )
+    total = (
+        space_and_water + electric_shower_co2 + space_cooling_co2
+        + pumps_fans_co2 + lighting_co2 - pv_credit
+    )
+    # (273) is rounded to 2 d.p. half-up — the PDF displays it with
+    # trailing zeros to 4 d.p. but precision is 2 d.p. throughout.
+    per_m2_raw = total / dim.total_floor_area_m2 if dim.total_floor_area_m2 > 0 else 0.0
+    per_m2 = _round_half_up(per_m2_raw, 2)
+    ei_continuous = environmental_impact_rating(
+        co2_emissions_kg_per_yr=total,
+        total_floor_area_m2=dim.total_floor_area_m2,
+    )
+    return EnvironmentalSection(
+        main_1_co2_kg_per_yr=main_1_co2,
+        main_2_co2_kg_per_yr=main_2_co2,
+        secondary_co2_kg_per_yr=secondary_co2,
+        water_heating_co2_kg_per_yr=water_co2,
+        electric_shower_co2_kg_per_yr=electric_shower_co2,
+        space_and_water_co2_kg_per_yr=space_and_water,
+        space_cooling_co2_kg_per_yr=space_cooling_co2,
+        pumps_fans_co2_kg_per_yr=pumps_fans_co2,
+        lighting_co2_kg_per_yr=lighting_co2,
+        pv_co2_credit_kg_per_yr=pv_credit,
+        total_co2_kg_per_yr=total,
+        co2_per_m2_kg_per_yr=per_m2,
+        ei_value_continuous=ei_continuous,
+        ei_rating_integer=max(1, round(ei_continuous)),
+    )
+
+
 def sap_rating_section_from_cert(
     epc: EpcPropertyData,
 ) -> Optional[SapRatingSection]:
@@ -1630,6 +1772,7 @@ def cert_to_inputs(
     # §5 cascade so the cost-side `inputs.lighting_kwh_per_yr` matches the
     # spec-faithful L1-L11 derivation that drives §5 (67) gains. Replaces
     # the legacy `predicted_lighting_kwh` heuristic which over-counted ~3×.
+    lighting_monthly_kwh: tuple[float, ...] = (0.0,) * 12
     if epc.total_floor_area_m2 is None:
         internal_gains_monthly_w = (0.0,) * 12
         lighting_kwh = 0.0
@@ -1645,6 +1788,13 @@ def cert_to_inputs(
             internal_gains_result.total_internal_gains_monthly_w
         )
         lighting_kwh = internal_gains_result.lighting_kwh_per_yr
+        # Watts → kWh via n_days_in_month × 24 hours / 1000 W per kWh.
+        lighting_monthly_kwh = tuple(
+            w * d * 24.0 / 1000.0
+            for w, d in zip(
+                internal_gains_result.lighting_monthly_w, _DAYS_IN_MONTH
+            )
+        )
 
     solar_gains_monthly_w = solar_gains_from_cert(
         epc=epc,
@@ -1828,6 +1978,34 @@ def cert_to_inputs(
             epc.sap_energy_source.meter_type, prices
         ),
         co2_factor_kg_per_kwh=_co2_factor_kg_per_kwh(main),
+        # SAP10.2 Table 12d (p.194) per-end-use effective CO2 factors. For
+        # electricity end-uses Σ(kWh_m × CO2_m) / Σ(kWh_m) replaces the
+        # annual-average Table 12 factor; gas end-uses pass through as the
+        # annual Table 12 value. None → calculator falls back to the global
+        # `co2_factor_kg_per_kwh`. Secondary heating defaults to standard
+        # electricity per RdSAP §A.2.2 (portable electric heater).
+        main_heating_co2_factor_kg_per_kwh=_co2_factor_kg_per_kwh(main),
+        secondary_heating_co2_factor_kg_per_kwh=_effective_monthly_co2_factor(
+            energy_requirements_result.secondary_fuel_monthly_kwh,
+            _STANDARD_ELECTRICITY_FUEL_CODE,
+        ),
+        hot_water_co2_factor_kg_per_kwh=co2_factor_kg_per_kwh(
+            epc.sap_heating.water_heating_fuel or main_fuel
+        ),
+        pumps_fans_co2_factor_kg_per_kwh=_effective_monthly_co2_factor(
+            _days_in_month_proportioned(pumps_fans_kwh, _DAYS_IN_MONTH),
+            _STANDARD_ELECTRICITY_FUEL_CODE,
+        ),
+        lighting_co2_factor_kg_per_kwh=_effective_monthly_co2_factor(
+            lighting_monthly_kwh, _STANDARD_ELECTRICITY_FUEL_CODE,
+        ),
+        electric_shower_kwh_per_yr=(
+            wh_result.electric_shower_kwh_per_yr if wh_result is not None else 0.0
+        ),
+        electric_shower_co2_factor_kg_per_kwh=_effective_monthly_co2_factor(
+            wh_result.electric_shower_monthly_kwh if wh_result is not None else (0.0,) * 12,
+            _STANDARD_ELECTRICITY_FUEL_CODE,
+        ),
         pv_generation_kwh_per_yr=_pv_generation_kwh_per_yr(epc),
         pv_export_credit_gbp_per_kwh=_pv_export_credit_gbp_per_kwh(prices),
         secondary_heating_fraction=secondary_fraction_value,
diff --git a/packages/domain/src/domain/sap/tables/table_12.py b/packages/domain/src/domain/sap/tables/table_12.py
index f4f72fb1..63b6af14 100644
--- a/packages/domain/src/domain/sap/tables/table_12.py
+++ b/packages/domain/src/domain/sap/tables/table_12.py
@@ -19,7 +19,7 @@ The Energy Cost Deflator stays at 0.36 (used in ECF — see
 
 from __future__ import annotations
 
-from typing import Final
+from typing import Final, Optional
 
 
 # SAP 10.3 Table 12 — unit price in pence per kWh.
@@ -77,10 +77,56 @@ UNIT_PRICE_P_PER_KWH: Final[dict[int, float]] = {
 _DEFAULT_P_PER_KWH: Final[float] = 3.64  # fall back to mains gas
 
 
-# SAP 10.2 Table 12 — CO2 emission factor in kg CO2-equivalent per kWh
-# of delivered energy. Grid electricity uses the annual-average 0.136
-# kg/kWh; the monthly factors in Table 12d are for comparison only per
-# note (s).
+# SAP 10.2 Table 12 — annual-average CO2 emission factor in kg CO2-
+# equivalent per kWh of delivered energy. For ELECTRICITY end-uses,
+# Table 12d (above) overrides this annual factor with monthly values per
+# the spec text on p.194; the value here is the legacy fallback when
+# monthly distribution isn't available.
+# SAP 10.2 Table 12d (p.194) — monthly variation in CO2 emission factors
+# for electricity. The spec text: "Where electricity is the fuel used, the
+# relevant set of factors in the table below should be used to calculate
+# the monthly CO2 emissions INSTEAD of the annual average factor given in
+# Table 12." So for ratings, electricity end-uses use Σ(kWh_m × CO2_m)
+# rather than annual_kwh × annual_factor.
+CO2_KG_PER_KWH_MONTHLY: Final[dict[int, tuple[float, ...]]] = {
+    # Standard tariff (default electricity)
+    30: (0.163, 0.160, 0.153, 0.143, 0.132, 0.120, 0.111, 0.112, 0.122, 0.136, 0.151, 0.163),
+    # 7-hour tariff
+    32: (0.171, 0.168, 0.161, 0.150, 0.138, 0.125, 0.117, 0.118, 0.128, 0.143, 0.158, 0.171),
+    31: (0.143, 0.141, 0.135, 0.126, 0.116, 0.105, 0.098, 0.099, 0.107, 0.120, 0.133, 0.144),
+    # 10-hour tariff
+    34: (0.168, 0.165, 0.159, 0.148, 0.136, 0.124, 0.115, 0.116, 0.126, 0.141, 0.156, 0.168),
+    33: (0.155, 0.153, 0.146, 0.137, 0.126, 0.114, 0.106, 0.107, 0.116, 0.130, 0.144, 0.155),
+    # 18-hour tariff (matches standard tariff)
+    38: (0.163, 0.160, 0.153, 0.143, 0.132, 0.120, 0.111, 0.112, 0.122, 0.136, 0.151, 0.163),
+    40: (0.163, 0.160, 0.153, 0.143, 0.132, 0.120, 0.111, 0.112, 0.122, 0.136, 0.151, 0.163),
+    # 24-hour heating tariff
+    35: (0.163, 0.160, 0.153, 0.143, 0.132, 0.120, 0.111, 0.112, 0.122, 0.136, 0.151, 0.163),
+    # Electricity sold to grid (PV)
+    60: (0.196, 0.190, 0.175, 0.153, 0.129, 0.106, 0.092, 0.093, 0.110, 0.138, 0.169, 0.197),
+    # Electricity sold to grid, other
+    36: (0.163, 0.160, 0.153, 0.143, 0.132, 0.120, 0.111, 0.112, 0.122, 0.136, 0.151, 0.163),
+    # Electricity, any tariff
+    39: (0.163, 0.160, 0.153, 0.143, 0.132, 0.120, 0.111, 0.112, 0.122, 0.136, 0.151, 0.163),
+    # Heat from electric heat pump
+    41: (0.163, 0.160, 0.153, 0.143, 0.132, 0.120, 0.111, 0.112, 0.122, 0.136, 0.151, 0.163),
+    # Low-grade heat recovered from process
+    49: (0.163, 0.160, 0.153, 0.143, 0.132, 0.120, 0.111, 0.112, 0.122, 0.136, 0.151, 0.163),
+    # Electricity for pumping in distribution network
+    50: (0.163, 0.160, 0.153, 0.143, 0.132, 0.120, 0.111, 0.112, 0.122, 0.136, 0.151, 0.163),
+}
+
+
+def co2_monthly_factors_kg_per_kwh(fuel_code: int | None) -> Optional[tuple[float, ...]]:
+    """SAP 10.2 Table 12d (p.194) monthly CO2 factors for electricity. Returns
+    None for non-electricity fuels (use the annual `co2_factor_kg_per_kwh`)."""
+    if fuel_code is None:
+        return None
+    if fuel_code in CO2_KG_PER_KWH_MONTHLY:
+        return CO2_KG_PER_KWH_MONTHLY[fuel_code]
+    return None
+
+
 CO2_KG_PER_KWH: Final[dict[int, float]] = {
     # Gas fuels
     1: 0.210,
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 4997a70b..0b6f6aea 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
@@ -496,3 +496,21 @@ LINE_256_ENERGY_COST_DEFLATOR: float = 0.4200
 LINE_257_ECF: float = 2.7055
 SAP_VALUE_CONTINUOUS: float = 62.2584
 LINE_258_SAP_RATING_INTEGER: int = 62
+
+# ============================================================================
+# §12 Carbon dioxide emissions — line refs (261)..(274)
+# ============================================================================
+LINE_261_MAIN_1_CO2: float = 2512.6274
+LINE_262_MAIN_2_CO2: float = 0.0
+LINE_263_SECONDARY_CO2: float = 0.0
+LINE_264_WATER_CO2: float = 481.2735
+LINE_264A_ELECTRIC_SHOWER_CO2: float = 0.0
+LINE_265_SPACE_AND_WATER_CO2: float = 2993.9009
+LINE_266_COOLING_CO2: float = 0.0
+LINE_267_PUMPS_FANS_CO2: float = 22.1940
+LINE_268_LIGHTING_CO2: float = 20.1984
+LINE_269_PV_CO2_CREDIT: float = 0.0
+LINE_272_TOTAL_CO2: float = 3036.2933
+LINE_273_CO2_PER_M2: float = 53.4700
+EI_VALUE_CONTINUOUS: float = 59.9093
+LINE_274_EI_RATING_INTEGER: int = 60
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
index 028b7518..dc7b2bb7 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
@@ -463,3 +463,21 @@ LINE_256_ENERGY_COST_DEFLATOR: float = 0.4200
 LINE_257_ECF: float = 2.5086
 SAP_VALUE_CONTINUOUS: float = 65.0057
 LINE_258_SAP_RATING_INTEGER: int = 65
+
+# ============================================================================
+# §12 Carbon dioxide emissions — line refs (261)..(274)
+# ============================================================================
+LINE_261_MAIN_1_CO2: float = 2156.9042
+LINE_262_MAIN_2_CO2: float = 0.0
+LINE_263_SECONDARY_CO2: float = 155.2882
+LINE_264_WATER_CO2: float = 444.3677
+LINE_264A_ELECTRIC_SHOWER_CO2: float = 0.0
+LINE_265_SPACE_AND_WATER_CO2: float = 2756.5602
+LINE_266_COOLING_CO2: float = 0.0
+LINE_267_PUMPS_FANS_CO2: float = 22.1940
+LINE_268_LIGHTING_CO2: float = 29.1080
+LINE_269_PV_CO2_CREDIT: float = 0.0
+LINE_272_TOTAL_CO2: float = 2807.8621
+LINE_273_CO2_PER_M2: float = 36.1900
+EI_VALUE_CONTINUOUS: float = 69.3055
+LINE_274_EI_RATING_INTEGER: int = 69
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py
index e22ef452..9b8a2a62 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py
@@ -505,3 +505,21 @@ LINE_256_ENERGY_COST_DEFLATOR: float = 0.4200
 LINE_257_ECF: float = 2.7743
 SAP_VALUE_CONTINUOUS: float = 61.2986
 LINE_258_SAP_RATING_INTEGER: int = 61
+
+# ============================================================================
+# §12 Carbon dioxide emissions — line refs (261)..(274)
+# ============================================================================
+LINE_261_MAIN_1_CO2: float = 2641.8617
+LINE_262_MAIN_2_CO2: float = 0.0
+LINE_263_SECONDARY_CO2: float = 190.1873
+LINE_264_WATER_CO2: float = 508.9643
+LINE_264A_ELECTRIC_SHOWER_CO2: float = 0.0
+LINE_265_SPACE_AND_WATER_CO2: float = 3341.0133
+LINE_266_COOLING_CO2: float = 0.0
+LINE_267_PUMPS_FANS_CO2: float = 22.1940
+LINE_268_LIGHTING_CO2: float = 30.6780
+LINE_269_PV_CO2_CREDIT: float = 0.0
+LINE_272_TOTAL_CO2: float = 3393.8852
+LINE_273_CO2_PER_M2: float = 40.2100
+EI_VALUE_CONTINUOUS: float = 64.8574
+LINE_274_EI_RATING_INTEGER: int = 65
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
index da027d5c..b7120ea2 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
@@ -529,3 +529,22 @@ LINE_256_ENERGY_COST_DEFLATOR: float = 0.4200
 LINE_257_ECF: float = 2.7496
 SAP_VALUE_CONTINUOUS: float = 61.6431
 LINE_258_SAP_RATING_INTEGER: int = 62
+
+# ============================================================================
+# §12 Carbon dioxide emissions — line refs (261)..(274)
+# ============================================================================
+# 000487 has electric shower: (264a) is non-zero.
+LINE_261_MAIN_1_CO2: float = 2313.8678
+LINE_262_MAIN_2_CO2: float = 0.0
+LINE_263_SECONDARY_CO2: float = 166.2086
+LINE_264_WATER_CO2: float = 312.7117
+LINE_264A_ELECTRIC_SHOWER_CO2: float = 83.6458
+LINE_265_SPACE_AND_WATER_CO2: float = 2792.7881
+LINE_266_COOLING_CO2: float = 0.0
+LINE_267_PUMPS_FANS_CO2: float = 22.1940
+LINE_268_LIGHTING_CO2: float = 32.8621
+LINE_269_PV_CO2_CREDIT: float = 0.0
+LINE_272_TOTAL_CO2: float = 2931.4900
+LINE_273_CO2_PER_M2: float = 35.9400
+EI_VALUE_CONTINUOUS: float = 68.9642
+LINE_274_EI_RATING_INTEGER: int = 69
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 40cb5c91..a1b092fa 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
@@ -479,3 +479,21 @@ LINE_256_ENERGY_COST_DEFLATOR: float = 0.4200
 LINE_257_ECF: float = 3.0539
 SAP_VALUE_CONTINUOUS: float = 57.3979
 LINE_258_SAP_RATING_INTEGER: int = 57
+
+# ============================================================================
+# §12 Carbon dioxide emissions — line refs (261)..(274)
+# ============================================================================
+LINE_261_MAIN_1_CO2: float = 2396.4161
+LINE_262_MAIN_2_CO2: float = 0.0
+LINE_263_SECONDARY_CO2: float = 171.5647
+LINE_264_WATER_CO2: float = 598.6197
+LINE_264A_ELECTRIC_SHOWER_CO2: float = 0.0
+LINE_265_SPACE_AND_WATER_CO2: float = 3166.6005
+LINE_266_COOLING_CO2: float = 0.0
+LINE_267_PUMPS_FANS_CO2: float = 22.1940
+LINE_268_LIGHTING_CO2: float = 24.7414
+LINE_269_PV_CO2_CREDIT: float = 0.0
+LINE_272_TOTAL_CO2: float = 3213.5359
+LINE_273_CO2_PER_M2: float = 48.6500
+EI_VALUE_CONTINUOUS: float = 61.1646
+LINE_274_EI_RATING_INTEGER: int = 61
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py
index b084d298..1089c179 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py
@@ -505,3 +505,21 @@ LINE_256_ENERGY_COST_DEFLATOR: float = 0.4200
 LINE_257_ECF: float = 2.6671
 SAP_VALUE_CONTINUOUS: float = 62.7937
 LINE_258_SAP_RATING_INTEGER: int = 63
+
+# ============================================================================
+# §12 Carbon dioxide emissions — line refs (261)..(274)
+# ============================================================================
+LINE_261_MAIN_1_CO2: float = 2647.3475
+LINE_262_MAIN_2_CO2: float = 0.0
+LINE_263_SECONDARY_CO2: float = 190.4096
+LINE_264_WATER_CO2: float = 523.5699
+LINE_264A_ELECTRIC_SHOWER_CO2: float = 0.0
+LINE_265_SPACE_AND_WATER_CO2: float = 3361.3270
+LINE_266_COOLING_CO2: float = 0.0
+LINE_267_PUMPS_FANS_CO2: float = 22.1940
+LINE_268_LIGHTING_CO2: float = 33.3239
+LINE_269_PV_CO2_CREDIT: float = 0.0
+LINE_272_TOTAL_CO2: float = 3416.8449
+LINE_273_CO2_PER_M2: float = 37.7400
+EI_VALUE_CONTINUOUS: float = 66.2198
+LINE_274_EI_RATING_INTEGER: int = 66
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_section_cascade_pins.py b/packages/domain/src/domain/sap/worksheet/tests/test_section_cascade_pins.py
index f1bfe4b7..6f52080f 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_section_cascade_pins.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_section_cascade_pins.py
@@ -19,6 +19,7 @@ import pytest
 from domain.sap.rdsap.cert_to_inputs import (
     cert_to_inputs,
     energy_requirements_section_from_cert,
+    environmental_section_from_cert,
     fabric_energy_efficiency_from_cert,
     fuel_cost_section_from_cert,
     heat_transmission_section_from_cert,
@@ -891,3 +892,55 @@ def test_section_11a_line_refs_match_pdf(
 
     # Assert
     _pin(actual, expected, f"§11a {fixture_attr} {fixture_name}")
+
+
+# ============================================================================
+# §12 Environmental (CO2 emissions) — LINE_261..LINE_274
+# ============================================================================
+
+_SECTION_12_PINS: Final[tuple[tuple[str, str], ...]] = (
+    ("LINE_261_MAIN_1_CO2",             "main_1_co2_kg_per_yr"),
+    ("LINE_262_MAIN_2_CO2",             "main_2_co2_kg_per_yr"),
+    ("LINE_263_SECONDARY_CO2",          "secondary_co2_kg_per_yr"),
+    ("LINE_264_WATER_CO2",              "water_heating_co2_kg_per_yr"),
+    ("LINE_264A_ELECTRIC_SHOWER_CO2",   "electric_shower_co2_kg_per_yr"),
+    ("LINE_265_SPACE_AND_WATER_CO2",    "space_and_water_co2_kg_per_yr"),
+    ("LINE_266_COOLING_CO2",            "space_cooling_co2_kg_per_yr"),
+    ("LINE_267_PUMPS_FANS_CO2",         "pumps_fans_co2_kg_per_yr"),
+    ("LINE_268_LIGHTING_CO2",           "lighting_co2_kg_per_yr"),
+    ("LINE_269_PV_CO2_CREDIT",          "pv_co2_credit_kg_per_yr"),
+    ("LINE_272_TOTAL_CO2",              "total_co2_kg_per_yr"),
+    ("LINE_273_CO2_PER_M2",             "co2_per_m2_kg_per_yr"),
+    ("EI_VALUE_CONTINUOUS",             "ei_value_continuous"),
+    ("LINE_274_EI_RATING_INTEGER",      "ei_rating_integer"),
+)
+
+
+@pytest.mark.parametrize(
+    "fixture_name,fixture_attr,result_attr",
+    [
+        (fix, line, attr)
+        for fix in _FIXTURES
+        for line, attr in _SECTION_12_PINS
+    ],
+    ids=lambda x: x if isinstance(x, str) else None,
+)
+def test_section_12_line_refs_match_pdf(
+    fixture_name: str, fixture_attr: str, result_attr: str
+) -> None:
+    """§12 pins — every (261)..(274) line ref of `environmental_section_
+    from_cert` matches the U985 PDF to abs=1e-4. Electricity end-uses use
+    Table 12d (p.194) monthly cascade Σ(kWh_m × CO2_m); gas end-uses use
+    the annual Table 12 factor 0.21."""
+    # Arrange
+    mod = _FIXTURES[fixture_name]
+    epc = mod.build_epc()  # type: ignore[attr-defined]
+    expected = getattr(mod, fixture_attr)
+
+    # Act
+    env = environmental_section_from_cert(epc)
+    assert env is not None, f"{fixture_name}: env_from_cert returned None"
+    actual = getattr(env, result_attr)
+
+    # Assert
+    _pin(actual, expected, f"§12 {fixture_attr} {fixture_name}")