diff --git a/docs/adr/0019-wall-insulation-eligibility.md b/docs/adr/0019-wall-insulation-eligibility.md
index 1b46f165..937ce79f 100644
--- a/docs/adr/0019-wall-insulation-eligibility.md
+++ b/docs/adr/0019-wall-insulation-eligibility.md
@@ -4,7 +4,9 @@ The solid-wall Recommendation Generator must decide, per Property, which wall-in
 
 ## Decision
 
-**By construction** (keyed on the `wall_construction` code, which is consistent across the API and Elmhurst paths for codes 1-5; the wall *description* is empty on the Elmhurst ingestion path so it can't be the primary signal — it's a fallback for the ambiguous higher codes (system-built 6-vs-8, cob 7) and for refining the as-built trigger on the API path):
+**By construction** (keyed on the `wall_construction` code, which is consistent across the API and Elmhurst paths for codes 1-5; the wall *description* is empty on the Elmhurst ingestion path so it can't be the primary signal — it's a fallback for the ambiguous codes (cob 7) and for refining the as-built trigger on the API path).
+
+**System-built** is keyed on `wall_construction == 6` (`WALL_SYSTEM_BUILT`; the Elmhurst `SY System build` label). This code is currently *overloaded*: `B Basement wall` also maps to 6 (`BASEMENT_WALL_CONSTRUCTION_CODE`, `mapper.py:2100`), so the generator additionally guards on `main_wall_is_basement` — a basement wall is never solid-wall-insulation-suitable and is excluded regardless of construction. Because `main_wall_is_basement` is presently derived as `wall_construction == 6`, *every* code-6 wall is treated as basement today, so the system-built branch is inert until the calculator disambiguates system-built from basement (target: MAIN `wall_construction == 6` with `main_wall_is_basement` False). The strict-xfail pin `test_system_built_generator_offers_ewi_and_iwi_each_pinning_its_after` is the tripwire for that fix. Note `wall_construction == 8` is **Park home** (`PH`) on the Elmhurst path, *not* system-built — do not key system-built on 8.
 
 | Construction | Cavity fill | IWI | EWI |
 |---|---|---|---|
diff --git a/domain/modelling/generators/solid_wall_recommendation.py b/domain/modelling/generators/solid_wall_recommendation.py
index 9da36bb1..9d4a67e0 100644
--- a/domain/modelling/generators/solid_wall_recommendation.py
+++ b/domain/modelling/generators/solid_wall_recommendation.py
@@ -34,6 +34,12 @@ _INTERNAL_MEASURE_TYPE: Final[str] = "internal_wall_insulation"
 # RdSAP `wall_construction` codes (consistent across paths for 1-5).
 _WALL_SOLID_BRICK: Final[int] = 3
 _WALL_TIMBER_FRAME: Final[int] = 5
+# System-built (precast/no-fines concrete): `WALL_SYSTEM_BUILT` in
+# rdsap_uvalues. NB this is the Elmhurst code (`SY`); the *basement-wall* signal
+# also lodges as 6 today (`BASEMENT_WALL_CONSTRUCTION_CODE`), so system-built is
+# disambiguated from basement by `main_wall_is_basement` below — a basement wall
+# is never solid-wall-insulation-suitable regardless.
+_WALL_SYSTEM_BUILT: Final[int] = 6
 # `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.
@@ -44,10 +50,11 @@ _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 takes IWI only (EWI not constructable). System-built
-# and the breathable cob/stone exclusions land in a later slice.
+# and system-built take both; timber-frame takes IWI only (EWI not
+# constructable). The breathable cob/stone exclusions take neither (never keyed).
 _CONSTRUCTABLE_OPTIONS: Final[dict[int, tuple[str, ...]]] = {
     _WALL_SOLID_BRICK: (_EXTERNAL_MEASURE_TYPE, _INTERNAL_MEASURE_TYPE),
+    _WALL_SYSTEM_BUILT: (_EXTERNAL_MEASURE_TYPE, _INTERNAL_MEASURE_TYPE),
     _WALL_TIMBER_FRAME: (_INTERNAL_MEASURE_TYPE,),
 }
 
@@ -129,6 +136,9 @@ def recommend_solid_wall(
     if main.wall_insulation_type != _WALL_AS_BUILT:
         return None
 
+    if main.main_wall_is_basement:
+        return None  # a (below-ground) basement wall is never EWI/IWI-suitable
+
     construction: object = main.wall_construction
     if not isinstance(construction, int):
         return None  # a free-text site-notes construction is not a code we key on
diff --git a/tests/domain/modelling/fixtures/system_built_ewi_001431_after.pdf b/tests/domain/modelling/fixtures/system_built_ewi_001431_after.pdf
new file mode 100644
index 00000000..1b76e032
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diff --git a/tests/domain/modelling/fixtures/system_built_ewi_001431_before.pdf b/tests/domain/modelling/fixtures/system_built_ewi_001431_before.pdf
new file mode 100644
index 00000000..0e4830b1
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diff --git a/tests/domain/modelling/fixtures/system_built_iwi_001431_after.pdf b/tests/domain/modelling/fixtures/system_built_iwi_001431_after.pdf
new file mode 100644
index 00000000..c943c608
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diff --git a/tests/domain/modelling/fixtures/system_built_iwi_001431_before.pdf b/tests/domain/modelling/fixtures/system_built_iwi_001431_before.pdf
new file mode 100644
index 00000000..0e4830b1
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diff --git a/tests/domain/modelling/test_elmhurst_cascade_pins.py b/tests/domain/modelling/test_elmhurst_cascade_pins.py
index 731e352d..97fccc84 100644
--- a/tests/domain/modelling/test_elmhurst_cascade_pins.py
+++ b/tests/domain/modelling/test_elmhurst_cascade_pins.py
@@ -17,6 +17,8 @@ from __future__ import annotations
 from dataclasses import replace
 from typing import Final
 
+import pytest
+
 from datatypes.epc.domain.epc_property_data import (
     BuildingPartIdentifier,
     EpcPropertyData,
@@ -173,6 +175,45 @@ def test_solid_brick_generator_offers_ewi_and_iwi_each_pinning_its_after() -> No
     )
 
 
+@pytest.mark.xfail(
+    strict=True,
+    reason="Blocked on the calculator-side wall_construction=6 collision: "
+    "Elmhurst 'SY System build' and 'B Basement wall' both map to 6 "
+    "(mapper.py:2100), so `main_wall_is_basement` is wrongly True for "
+    "system-built and the generator's basement guard suppresses it. Flips "
+    "green once system-built is disambiguated from basement (MAIN "
+    "wall_construction==6 with main_wall_is_basement False).",
+)
+def test_system_built_generator_offers_ewi_and_iwi_each_pinning_its_after() -> None:
+    # Arrange — system-built (precast concrete) takes both Options like solid
+    # brick (ADR-0019): one uninsulated before, two re-lodged afters.
+    before: EpcPropertyData = parse_recommendation_summary(
+        "system_built_ewi_001431_before.pdf"
+    )
+    ewi_after: EpcPropertyData = parse_recommendation_summary(
+        "system_built_ewi_001431_after.pdf"
+    )
+    iwi_after: EpcPropertyData = parse_recommendation_summary(
+        "system_built_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 — both Options offered, each reproducing its own re-lodged after.
+    assert set(options) == {"external_wall_insulation", "internal_wall_insulation"}
+    _assert_overlay_reproduces_after(
+        before, ewi_after, options["external_wall_insulation"].overlay
+    )
+    _assert_overlay_reproduces_after(
+        before, iwi_after, options["internal_wall_insulation"].overlay
+    )
+
+
 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(