diff --git a/tests/applications/modelling_e2e/test_handler.py b/tests/applications/modelling_e2e/test_handler.py
index f2e33187b..32319bcd5 100644
--- a/tests/applications/modelling_e2e/test_handler.py
+++ b/tests/applications/modelling_e2e/test_handler.py
@@ -511,6 +511,70 @@ def test_attach_mode_partial_failure_persists_successes_then_records_failure() -
     ]
 
 
+def test_first_plan_for_a_property_becomes_its_default() -> None:
+    """A property's first Plan is its default whatever the Scenario says (the
+    legacy p.is_new rule — readers select one default Plan per property); a
+    property that already has a default Plan keeps the Scenario's flag."""
+    # Arrange — a non-default scenario; 111 has no default plan yet, 222 does
+    pid_first, pid_has_default = 111, 222
+    mock_engine = _engine_mock(
+        [pid_first, pid_has_default], [1001, 1002], [POSTCODE, POSTCODE]
+    )
+    scenario = MagicMock()
+    scenario.is_default = False
+
+    with ExitStack() as stack:
+        stack.enter_context(patch("applications.modelling_e2e.handler.os.environ", _ENV))
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler._get_engine", return_value=mock_engine)
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.EpcClientService")
+        ).return_value.get_by_uprn.return_value = MagicMock()
+        stack.enter_context(patch("applications.modelling_e2e.handler.GeospatialS3Repository"))
+        stack.enter_context(patch("applications.modelling_e2e.handler.GoogleSolarApiClient"))
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler._spatial_for", return_value=None)
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler._solar_insights_for", return_value=None)
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.overlays_from", return_value=[])
+        )
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.PropertyOverridesPostgresReader")
+        ).return_value.overrides_for_many.return_value = {}
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.ScenarioPostgresRepository")
+        ).return_value.get_many.return_value = [scenario]
+        stack.enter_context(patch("applications.modelling_e2e.handler.catalogue_snapshot_with_off_catalogue_overrides"))
+        stack.enter_context(patch("applications.modelling_e2e.handler.Session"))
+        stack.enter_context(
+            patch("applications.modelling_e2e.handler.run_modelling", return_value=_plan_mock())
+        )
+        MockUoW = stack.enter_context(patch("applications.modelling_e2e.handler.PostgresUnitOfWork"))
+        mock_uow = MagicMock()
+        mock_uow.plan.property_ids_with_default_plans.return_value = {pid_has_default}
+        MockUoW.return_value.__enter__.return_value = mock_uow
+        MockUoW.return_value.__exit__.return_value = False
+
+        # Act
+        from applications.modelling_e2e.handler import handler
+        handler.__wrapped__(  # type: ignore[attr-defined]
+            {"property_ids": [pid_first, pid_has_default],
+             "portfolio_id": PORTFOLIO_ID, "scenario_id": SCENARIO_ID,
+             "refetch_solar": False, "dry_run": False},
+            None, _mock_orchestrator(), uuid4(),
+        )
+
+    # Assert — the first-timer's plan is promoted; the other keeps the flag
+    plan_requests = mock_uow.plan.save_batch.call_args.args[0]
+    by_pid = {r.property_id: r for r in plan_requests}
+    assert by_pid[pid_first].is_default is True
+    assert by_pid[pid_has_default].is_default is False
+
+
 # ---------------------------------------------------------------------------
 # Lodged EPC path
 # ---------------------------------------------------------------------------