diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 0102e5f3..7dd3b3c2 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2,7 +2,7 @@ import re
 from dataclasses import replace
 from datetime import date
 from decimal import ROUND_HALF_UP, Decimal
-from typing import Any, Dict, Final, List, Optional, Sequence, Union, cast
+from typing import Any, Dict, Final, List, Optional, Sequence, Tuple, Union, cast
 from datatypes.epc.schema.helpers import from_dict
 
 from datatypes.epc.domain.epc_property_data import (
@@ -51,6 +51,7 @@ from datatypes.epc.schema.sap_schema_17_1 import (
 
 # full-SAP opening-type codes: 1/2/3 = door, 4 = window, 5 = roof window.
 _SAP_OPENING_TYPE_WINDOW: Final[int] = 4
+_SAP_OPENING_TYPE_DOORS: Final[frozenset[int]] = frozenset({1, 2, 3})
 # SAP-typical glazing solar transmittance when an opening-type omits it.
 _SAP_DEFAULT_SOLAR_TRANSMITTANCE: Final[float] = 0.63
 from datatypes.epc.schema.rdsap_schema_18_0 import (
@@ -632,10 +633,10 @@ class EpcPropertyDataMapper:
     @staticmethod
     def from_sap_schema_17_1(schema: SapSchema17_1) -> EpcPropertyData:
         # Built incrementally via TDD — see scripts/hyde/mapping_decisions.md.
-        # Tracer slice: identity + scalars are mapped; the load-bearing
-        # collections (walls/floors/roofs, sap_windows, sap_building_parts,
-        # door/room counts) are left empty here and filled by later slices
-        # (D1 perimeter, D2 openings, D3 living-area, D4 fabric-U).
+        # Identity + scalars + fabric descriptions (D4) + openings (D2) are
+        # mapped; sap_building_parts (D1 perimeter) and habitable_rooms_count
+        # (D3 living-area) are still filled by later slices.
+        door_count, insulated_door_u = _sap_door_aggregates(schema)
         return EpcPropertyData(
             uprn=schema.uprn,
             dwelling_type=(
@@ -653,13 +654,16 @@ class EpcPropertyDataMapper:
             has_hot_water_cylinder=schema.has_hot_water_cylinder == "true",
             has_fixed_air_conditioning=schema.has_fixed_air_conditioning == "true",
             solar_water_heating=False,
-            door_count=0,
             wet_rooms_count=0,
             extensions_count=0,
             heated_rooms_count=0,
             open_chimneys_count=0,
             habitable_rooms_count=0,
-            insulated_door_count=0,
+            # D2: door openings (1/2/3) → counts + area-weighted U. New-build
+            # doors are treated insulated, so insulated_door_count == door_count.
+            door_count=door_count,
+            insulated_door_count=door_count,
+            insulated_door_u_value=insulated_door_u,
             cfl_fixed_lighting_bulbs_count=0,
             led_fixed_lighting_bulbs_count=0,
             incandescent_fixed_lighting_bulbs_count=0,
@@ -2384,6 +2388,31 @@ class EpcPropertyDataMapper:
 # ---------------------------------------------------------------------------
 
 
+def _sap_door_aggregates(schema: SapSchema17_1) -> Tuple[int, Optional[float]]:
+    """D2: collapse door openings (opening-type 1/2/3) onto the engine's door
+    model — a count and a single U-value. When a cert lodges doors with
+    differing U (≈5% of full-SAP certs), the U is area-weighted so the larger
+    door dominates the door heat loss. Returns (door_count, u_value); U is None
+    when no doors are lodged."""
+    types: Dict[Union[str, int], SapOpeningType_SAP_17_1] = {
+        ot.name: ot for ot in schema.sap_opening_types
+    }
+    count = 0
+    weighted_u_area = 0.0
+    total_area = 0.0
+    for bp in schema.sap_building_parts:
+        for op in bp.sap_openings:
+            ot = types.get(op.type)
+            if ot is None or ot.type not in _SAP_OPENING_TYPE_DOORS:
+                continue
+            count += 1
+            area = float(op.width) * float(op.height)
+            weighted_u_area += ot.u_value * area
+            total_area += area
+    u_value = weighted_u_area / total_area if total_area > 0 else None
+    return count, u_value
+
+
 def _clear_basement_flag_when_system_built(
     epc: EpcPropertyData,
 ) -> EpcPropertyData:
diff --git a/datatypes/epc/domain/tests/test_from_sap_schema.py b/datatypes/epc/domain/tests/test_from_sap_schema.py
index 0caff7b0..018e0cc6 100644
--- a/datatypes/epc/domain/tests/test_from_sap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_sap_schema.py
@@ -139,3 +139,29 @@ class TestFromSapSchema17_1Windows:
 
     def test_window_orientation_passes_through(self, sample: EpcPropertyData) -> None:
         assert sample.sap_windows[0].orientation == 8
+
+
+class TestFromSapSchema17_1Doors:
+    """Slice 4b (D2): door openings (opening-type 1/2/3) collapse onto the
+    engine's door_count + insulated_door_u_value. New-build doors are treated
+    insulated, so insulated_door_count == door_count."""
+
+    def _map(self, fixture: str) -> EpcPropertyData:
+        schema = from_dict(SapSchema17_1, load(fixture))
+        return EpcPropertyDataMapper.from_sap_schema_17_1(schema)
+
+    def test_single_door_count_and_u(self) -> None:
+        result = self._map("sap_17_1.json")
+        assert result.door_count == 1
+        assert result.insulated_door_u_value == 1.4
+
+    def test_doors_treated_insulated(self) -> None:
+        result = self._map("sap_17_1.json")
+        assert result.insulated_door_count == 1
+
+    def test_multi_door_u_is_area_weighted(self) -> None:
+        # Flat lodges 2 doors with distinct U (1.4 over 1.89 m², 1.8 over 2.12 m²)
+        # → area-weighted mean so the dominant door drives the door heat loss.
+        result = self._map("sap_17_1_flat.json")
+        assert result.door_count == 2
+        assert result.insulated_door_u_value == pytest.approx(1.6114713, abs=1e-6)