diff --git a/domain/modelling/generators/solid_wall_recommendation.py b/domain/modelling/generators/solid_wall_recommendation.py
index 14b84535..31e59403 100644
--- a/domain/modelling/generators/solid_wall_recommendation.py
+++ b/domain/modelling/generators/solid_wall_recommendation.py
@@ -29,8 +29,9 @@ from repositories.product.product_repository import ProductRepository
 _EXTERNAL_MEASURE_TYPE: Final[str] = "external_wall_insulation"
 _INTERNAL_MEASURE_TYPE: Final[str] = "internal_wall_insulation"
 
-# RdSAP `wall_construction` code for solid brick (consistent across paths).
+# RdSAP `wall_construction` codes (consistent across paths for 1-5).
 _WALL_SOLID_BRICK: Final[int] = 3
+_WALL_TIMBER_FRAME: Final[int] = 5
 # `wall_insulation_type`: 4 = as-built / assumed (uninsulated) — the trigger.
 _WALL_AS_BUILT: Final[int] = 4
 # `wall_insulation_type` the overlay lodges: 1 = external, 3 = internal.
@@ -41,10 +42,11 @@ _WALL_INSULATION_INTERNAL: Final[int] = 3
 _SOLID_WALL_INSULATION_MM: Final[int] = 100
 
 # Which solid-wall Options each construction can take (ADR-0019). Solid brick
-# takes both; timber-frame (IWI only), system-built, and the breathable
-# cob/stone exclusions land in later slices.
+# takes both; timber-frame takes IWI only (EWI not constructable). System-built
+# and the breathable cob/stone exclusions land in a later slice.
 _CONSTRUCTABLE_OPTIONS: Final[dict[int, tuple[str, ...]]] = {
     _WALL_SOLID_BRICK: (_EXTERNAL_MEASURE_TYPE, _INTERNAL_MEASURE_TYPE),
+    _WALL_TIMBER_FRAME: (_INTERNAL_MEASURE_TYPE,),
 }
 
 _INSULATION_TYPE: Final[dict[str, int]] = {
diff --git a/tests/domain/modelling/fixtures/timber_frame_iwi_001431_after.pdf b/tests/domain/modelling/fixtures/timber_frame_iwi_001431_after.pdf
new file mode 100644
index 00000000..c24ae877
Binary files /dev/null and b/tests/domain/modelling/fixtures/timber_frame_iwi_001431_after.pdf differ
diff --git a/tests/domain/modelling/fixtures/timber_frame_iwi_001431_before.pdf b/tests/domain/modelling/fixtures/timber_frame_iwi_001431_before.pdf
new file mode 100644
index 00000000..fec38abb
Binary files /dev/null and b/tests/domain/modelling/fixtures/timber_frame_iwi_001431_before.pdf differ
diff --git a/tests/domain/modelling/test_elmhurst_cascade_pins.py b/tests/domain/modelling/test_elmhurst_cascade_pins.py
index 929c6d70..3be44f56 100644
--- a/tests/domain/modelling/test_elmhurst_cascade_pins.py
+++ b/tests/domain/modelling/test_elmhurst_cascade_pins.py
@@ -171,6 +171,29 @@ def test_solid_brick_generator_offers_ewi_and_iwi_each_pinning_its_after() -> No
     )
 
 
+def test_timber_frame_generator_offers_iwi_only_pinning_its_after() -> None:
+    # Arrange — timber frame takes IWI but EWI is not constructable (ADR-0019).
+    before: EpcPropertyData = parse_recommendation_summary(
+        "timber_frame_iwi_001431_before.pdf"
+    )
+    iwi_after: EpcPropertyData = parse_recommendation_summary(
+        "timber_frame_iwi_001431_after.pdf"
+    )
+
+    # Act
+    recommendation: Recommendation | None = recommend_solid_wall(before, _AnyProduct())
+    assert recommendation is not None
+    options: dict[str, MeasureOption] = {
+        option.measure_type: option for option in recommendation.options
+    }
+
+    # Assert — IWI only, and it reproduces the re-lodged after.
+    assert set(options) == {"internal_wall_insulation"}
+    _assert_overlay_reproduces_after(
+        before, iwi_after, options["internal_wall_insulation"].overlay
+    )
+
+
 def test_loft_overlay_reproduces_the_relodged_after() -> None:
     # Arrange
     before: EpcPropertyData = parse_recommendation_summary(