diff --git a/src/grimp/adaptors/_layers.py b/src/grimp/adaptors/_layers.py
index f49daad4..b1cc630d 100644
--- a/src/grimp/adaptors/_layers.py
+++ b/src/grimp/adaptors/_layers.py
@@ -9,6 +9,7 @@
 
 if TYPE_CHECKING:
     from grimp.adaptors.graph import ImportGraph
+    from grimp.application.ports.graph import Level
 
 from grimp.domain.analysis import PackageDependency
 from grimp.exceptions import NoSuchContainer
@@ -16,7 +17,7 @@
 
 def find_illegal_dependencies(
     graph: ImportGraph,
-    layers: Sequence[Union[str, set[str]]],
+    layers: Sequence[Union[str, set[str], Level]],
     containers: set[str],
 ) -> set[PackageDependency]:
     """
@@ -52,11 +53,22 @@ class _RustPackageDependency(TypedDict):
     routes: tuple[_RustRoute, ...]
 
 
-def _layers_to_levels(layers: Sequence[Union[str, set[str]]]) -> tuple[set[str], ...]:
+def _layers_to_levels(layers: Sequence[Union[str, set[str], Level]]) -> tuple[Level, ...]:
     """
     Convert any standalone layers to a one-element level.
     """
-    return tuple({layer} if isinstance(layer, str) else set(layer) for layer in layers)
+    out_layers = []
+    for layer in layers:
+        if isinstance(layer, dict):
+            out_layers.append(dict(
+                independent=layer["independent"],
+                layers=set(layer["layers"]),
+            ))
+        if isinstance(layer, str):
+            out_layers.append(dict(independent=True, layers={layer}))
+        else:
+            out_layers.append(dict(independent=True, layers=set(layer)))
+    return tuple(out_layers)
 
 
 def _dependencies_from_tuple(
diff --git a/src/grimp/adaptors/graph.py b/src/grimp/adaptors/graph.py
index 7bc77743..0288a323 100644
--- a/src/grimp/adaptors/graph.py
+++ b/src/grimp/adaptors/graph.py
@@ -365,7 +365,7 @@ def chain_exists(self, importer: str, imported: str, as_packages: bool = False)
 
     def find_illegal_dependencies_for_layers(
         self,
-        layers: Sequence[str | set[str]],
+        layers: Sequence[str | set[str] | graph.Level],
         containers: Optional[set[str]] = None,
     ) -> set[PackageDependency]:
         return _layers.find_illegal_dependencies(
diff --git a/src/grimp/application/ports/graph.py b/src/grimp/application/ports/graph.py
index 68473516..272d77cd 100644
--- a/src/grimp/application/ports/graph.py
+++ b/src/grimp/application/ports/graph.py
@@ -15,6 +15,11 @@ class DetailedImport(TypedDict):
     line_contents: str
 
 
+class Level(TypedDict):
+    independent: bool
+    layers: set[str]
+
+
 class ImportGraph(abc.ABC):
     """
     A Directed Graph of imports between Python modules.
@@ -271,7 +276,7 @@ def chain_exists(self, importer: str, imported: str, as_packages: bool = False)
 
     def find_illegal_dependencies_for_layers(
         self,
-        layers: Sequence[Union[str, set[str]]],
+        layers: Sequence[Union[str, set[str], Level]],
         containers: Optional[set[str]] = None,
     ) -> set[PackageDependency]:
         """
diff --git a/tests/unit/adaptors/graph/test_layers.py b/tests/unit/adaptors/graph/test_layers.py
index 672af37b..4efd69a3 100644
--- a/tests/unit/adaptors/graph/test_layers.py
+++ b/tests/unit/adaptors/graph/test_layers.py
@@ -384,7 +384,6 @@ def _analyze(
                 layers=("mypackage.high", "mypackage.medium", "mypackage.low"),
             )
 
-
 class TestIndependentLayers:
     @pytest.mark.parametrize("specify_container", (True, False))
     def test_no_illegal_imports(self, specify_container: bool):
@@ -401,6 +400,9 @@ def test_no_illegal_imports(self, specify_container: bool):
             frozenset,
             tuple,
             list,
+            lambda layers: dict(independent=True, layers=set(layers)),
+            lambda layers: dict(independent=True, layers=tuple(layers)),
+            lambda layers: dict(independent=True, layers=list(layers)),
         ),
     )
     @pytest.mark.parametrize(
@@ -435,6 +437,24 @@ def test_direct_illegal_between_sibling_layers(
             ),
         }
 
+    def test_imports_between_sibling_layers_illegal_if_and_only_if_level_is_independent(self):
+        graph = self._build_legal_graph()
+        graph.add_import(importer="mypackage.foo", imported="mypackage.bar")
+
+        # Layer is independent => import is forbidden.
+        results = graph.find_illegal_dependencies_for_layers(
+            layers=("high", dict(independent=True, layers={"foo", "bar"}), "low"),
+            containers={"mypackage"},
+        )
+        assert results
+
+        # Layer is not independent => import is allowed.
+        results = graph.find_illegal_dependencies_for_layers(
+            layers=("high", dict(independent=False, layers={"foo", "bar"}), "low"),
+            containers={"mypackage"},
+        )
+        assert not results
+
     @pytest.mark.parametrize("specify_container", (True, False))
     @pytest.mark.parametrize(
         "start",