diff --git a/domain/epc/property_overrides/main_heating_guard.py b/domain/epc/property_overrides/main_heating_guard.py
new file mode 100644
index 000000000..fe8e54126
--- /dev/null
+++ b/domain/epc/property_overrides/main_heating_guard.py
@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.epc.property_overrides.main_heating_system_type import (
+    MainHeatingSystemType,
+)
+
+
+def main_heating_guard(description: str) -> Optional[MainHeatingSystemType]:
+    raise NotImplementedError
diff --git a/tests/domain/epc/test_main_heating_guard.py b/tests/domain/epc/test_main_heating_guard.py
new file mode 100644
index 000000000..452474d61
--- /dev/null
+++ b/tests/domain/epc/test_main_heating_guard.py
@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+import pytest
+
+from domain.epc.property_overrides.main_heating_guard import main_heating_guard
+from domain.epc.property_overrides.main_heating_system_type import (
+    MainHeatingSystemType,
+)
+
+
+def test_guard_resolves_high_heat_retention_storage() -> None:
+    # Arrange — the structured phrasing the LLM funnelled into "old storage" (401).
+    description = "Electric Storage Systems: High heat retention storage heaters"
+
+    # Act
+    result = main_heating_guard(description)
+
+    # Assert — HHRSH has its own archetype (SAP 409), never old storage (ADR-0044).
+    assert result is MainHeatingSystemType.ELECTRIC_STORAGE_HIGH_HEAT_RETENTION
+
+
+@pytest.mark.parametrize(
+    "description",
+    [
+        # Other storage subtypes the LLM already classifies correctly — the guard
+        # only rescues the HHRSH gap, leaving the rest to the fallback classifier.
+        "Electric Storage Systems: Old (large volume) storage heaters",
+        "Electric Storage Systems: Modern (slimline) storage heaters",
+        "Electric Storage Systems: Fan storage heaters",
+        # Unrelated / varied phrasings are the LLM's job.
+        "Gas boiler",
+        "",
+    ],
+)
+def test_guard_defers_non_hhrsh_descriptions_to_the_llm(description: str) -> None:
+    # Act
+    result = main_heating_guard(description)
+
+    # Assert
+    assert result is None