diff --git a/domain/epc/wall_type_overlay.py b/domain/epc/wall_type_overlay.py
new file mode 100644
index 00000000..edcb429a
--- /dev/null
+++ b/domain/epc/wall_type_overlay.py
@@ -0,0 +1,60 @@
+"""Map a Landlord-Override `WallType` value to a wall Simulation Overlay (ADR-0032).
+
+A `WallType` value is one full EPC wall description — *material* (cavity, solid
+brick, …) combined with *insulation state* (as built / with internal insulation
+/ filled cavity / …). The calculator scores the wall from the RdSAP
+`wall_construction` (material) and `wall_insulation_type` (state) **int codes**,
+never the description string, so the overlay decomposes the value into those two
+codes and emits an `EpcSimulation` targeting the override's building part. The
+result folds onto the lodged EPC via `apply_simulations`, exactly as a wall
+Measure's overlay does. Unresolvable material/state → None (no overlay).
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
+
+# RdSAP `wall_construction` codes by material prefix (domain/sap10_ml/rdsap_uvalues.py).
+_MATERIAL_CONSTRUCTION: dict[str, int] = {
+    "Cavity wall": 4,
+    "Solid brick": 3,
+}
+
+# RdSAP `wall_insulation_type` codes by insulation-state suffix
+# (domain/sap10_ml/rdsap_uvalues.py): external 1, filled-cavity 2, internal 3,
+# as-built/uninsulated 4, cavity+external 6, cavity+internal 7.
+_STATE_INSULATION: dict[str, int] = {
+    "as built, no insulation (assumed)": 4,
+    "with internal insulation": 3,
+    "with external insulation": 1,
+    "filled cavity": 2,
+    "filled cavity and internal insulation": 7,
+    "filled cavity and external insulation": 6,
+}
+
+
+def wall_overlay_for(
+    wall_type_value: str, building_part: int
+) -> Optional[EpcSimulation]:
+    material, _, state = wall_type_value.partition(", ")
+    construction = _MATERIAL_CONSTRUCTION.get(material)
+    insulation = _STATE_INSULATION.get(state)
+    if construction is None or insulation is None:
+        return None
+
+    identifier = (
+        BuildingPartIdentifier.MAIN
+        if building_part == 0
+        else BuildingPartIdentifier.extension(building_part)
+    )
+    return EpcSimulation(
+        building_parts={
+            identifier: BuildingPartOverlay(
+                wall_construction=construction,
+                wall_insulation_type=insulation,
+            )
+        }
+    )
diff --git a/domain/modelling/simulation.py b/domain/modelling/simulation.py
index 7d951ac5..c4708e7a 100644
--- a/domain/modelling/simulation.py
+++ b/domain/modelling/simulation.py
@@ -25,6 +25,10 @@ class BuildingPartOverlay:
     A `None` field means "leave the baseline value unchanged".
     """
 
+    # The wall material (RdSAP `wall_construction` code). Left `None` by Measures
+    # — insulating a wall doesn't change its material — but set by a Landlord
+    # Override that corrects the construction itself (ADR-0032).
+    wall_construction: Optional[int] = None
     wall_insulation_type: Optional[int] = None
     # Added solid-wall insulation depth (mm) — drives the calculator's Table 6
     # bucket / §5.8 documentary U-value for EWI (`wall_insulation_type=1`) and
diff --git a/tests/domain/epc/test_wall_type_overlay.py b/tests/domain/epc/test_wall_type_overlay.py
new file mode 100644
index 00000000..92c7a1f8
--- /dev/null
+++ b/tests/domain/epc/test_wall_type_overlay.py
@@ -0,0 +1,72 @@
+"""The Landlord-Override `WallType` → wall Simulation Overlay mapping (ADR-0032).
+
+A `WallType` value decomposes into the RdSAP `wall_construction` (material) and
+`wall_insulation_type` (state) int codes the calculator reads; the overlay
+targets the override's building part. Unresolvable values produce no overlay.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+from domain.epc.wall_type_overlay import wall_overlay_for
+
+
+def test_solid_brick_with_internal_insulation_overlays_main_wall() -> None:
+    # Act
+    simulation = wall_overlay_for("Solid brick, with internal insulation", 0)
+
+    # Assert — solid brick (wall_construction 3) + internal insulation (type 3)
+    # on the main building part.
+    assert simulation is not None
+    overlay = simulation.building_parts[BuildingPartIdentifier.MAIN]
+    assert overlay.wall_construction == 3
+    assert overlay.wall_insulation_type == 3
+
+
+@pytest.mark.parametrize(
+    ("wall_type_value", "construction", "insulation"),
+    [
+        ("Cavity wall, as built, no insulation (assumed)", 4, 4),
+        ("Cavity wall, with internal insulation", 4, 3),
+        ("Cavity wall, with external insulation", 4, 1),
+        ("Cavity wall, filled cavity", 4, 2),
+        ("Cavity wall, filled cavity and internal insulation", 4, 7),
+        ("Cavity wall, filled cavity and external insulation", 4, 6),
+        ("Solid brick, as built, no insulation (assumed)", 3, 4),
+        ("Solid brick, with external insulation", 3, 1),
+    ],
+)
+def test_material_and_state_decompose_to_their_gov_codes(
+    wall_type_value: str, construction: int, insulation: int
+) -> None:
+    # Act
+    simulation = wall_overlay_for(wall_type_value, 0)
+
+    # Assert
+    assert simulation is not None
+    overlay = simulation.building_parts[BuildingPartIdentifier.MAIN]
+    assert overlay.wall_construction == construction
+    assert overlay.wall_insulation_type == insulation
+
+
+def test_overlay_targets_the_extension_building_part() -> None:
+    # Act — building_part 1 is the first extension.
+    simulation = wall_overlay_for("Solid brick, with internal insulation", 1)
+
+    # Assert
+    assert simulation is not None
+    assert BuildingPartIdentifier.EXTENSION_1 in simulation.building_parts
+
+
+@pytest.mark.parametrize(
+    "wall_type_value",
+    ["Unknown", "Granite or whin, as built, no insulation (assumed)", ""],
+)
+def test_unresolvable_wall_type_produces_no_overlay(wall_type_value: str) -> None:
+    # Act
+    simulation = wall_overlay_for(wall_type_value, 0)
+
+    # Assert
+    assert simulation is None