diff --git a/rockcraft/commands/extensions.py b/rockcraft/commands/extensions.py
index 9870bde96..aef8946d8 100644
--- a/rockcraft/commands/extensions.py
+++ b/rockcraft/commands/extensions.py
@@ -97,6 +97,7 @@ class ExpandExtensionsCommand(AppCommand, abc.ABC):
     @overrides
     def run(self, parsed_args: argparse.Namespace) -> None:
         """Print the project's specification with the extensions expanded."""
-        project = Project.unmarshal(load_project(Path("rockcraft.yaml")))
+        project_path = Path("rockcraft.yaml")
+        project = Project.from_yaml_data(load_project(project_path), project_path)
 
         emit.message(project.to_yaml())  # pylint: disable=no-member
diff --git a/tests/unit/commands/test_expand_extensions.py b/tests/unit/commands/test_expand_extensions.py
index 365ee8244..d1c4e15ac 100644
--- a/tests/unit/commands/test_expand_extensions.py
+++ b/tests/unit/commands/test_expand_extensions.py
@@ -14,14 +14,22 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 import argparse
+import copy
+import re
 import textwrap
 from pathlib import Path
 
 import pytest
+from craft_application import util, errors
+
 from rockcraft import extensions
 from rockcraft.commands import ExpandExtensionsCommand
 
-from tests.unit.testing.extensions import FULL_EXTENSION_YAML, FullExtension
+from tests.unit.testing.extensions import (
+    FULL_EXTENSION_YAML,
+    FullExtension,
+    FULL_EXTENSION_PROJECT,
+)
 
 # The project with the extension (FullExtension) expanded
 EXPECTED_EXPAND_EXTENSIONS = textwrap.dedent(
@@ -82,3 +90,29 @@ def test_expand_extensions(setup_extensions, emitter, new_dir):
     cmd.run(argparse.Namespace())
 
     emitter.assert_message(EXPECTED_EXPAND_EXTENSIONS)
+
+
+def test_expand_extensions_error(setup_extensions, new_dir):
+    wrong_yaml = copy.deepcopy(FULL_EXTENSION_PROJECT)
+    # Set a wrong base
+    # wrong_yaml["base"] = "ubuntu@invalid"
+
+    # Mix slices and debs
+    wrong_yaml["parts"]["foo"]["stage-packages"].append("some-chisel_slice")
+
+    # Misconfigure a service
+    wrong_yaml["services"]["my-service"]["override"] = "invalid"
+
+    project_file = Path("rockcraft.yaml")
+    dumped = util.dump_yaml(wrong_yaml)
+    project_file.write_text(dumped)
+
+    expected_message = re.escape(
+        "Bad rockcraft.yaml content:\n"
+        "- cannot mix packages and slices in stage-packages (in field 'parts.foo')\n"
+        "- unexpected value; permitted: 'merge', 'replace' (in field 'services.my-service.override')"
+    )
+
+    cmd = ExpandExtensionsCommand(None)
+    with pytest.raises(errors.CraftValidationError, match=expected_message):
+        cmd.run(argparse.Namespace())
diff --git a/tests/unit/testing/extensions.py b/tests/unit/testing/extensions.py
index fed18152b..e6b8d269c 100644
--- a/tests/unit/testing/extensions.py
+++ b/tests/unit/testing/extensions.py
@@ -15,9 +15,9 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 """Fake Extensions for use in tests."""
-import textwrap
 from typing import Any
 
+from craft_application import util
 from overrides import override
 from rockcraft.extensions.extension import Extension
 
@@ -103,29 +103,23 @@ def get_parts_snippet(self) -> dict[str, Any]:
         return {"full-extension/new-part": {"plugin": "nil", "source": None}}
 
 
-FULL_EXTENSION_YAML = textwrap.dedent(
-    f"""
-    name: project-with-extensions
-    version: latest
-    base: ubuntu@22.04
-    summary: Project with extensions
-    description: Project with extensions
-    license: Apache-2.0
-    platforms:
-        amd64:
-
-    extensions:
-        - {FullExtension.NAME}
-
-    parts:
-        foo:
-            plugin: nil
-            stage-packages:
-                - old-package
-
-    services:
-        my-service:
-            command: foo
-            override: merge
-    """
-)
+FULL_EXTENSION_PROJECT = {
+    "name": "project-with-extensions",
+    "version": "latest",
+    "base": "ubuntu@22.04",
+    "summary": "Project with extensions",
+    "description": "Project with extensions",
+    "license": "Apache-2.0",
+    "platforms": {"amd64": None},
+    "extensions": [FullExtension.NAME],
+    "parts": {"foo": {"plugin": "nil", "stage-packages": ["old-package"]}},
+    "services": {
+        "my-service": {
+            "command": "foo",
+            "override": "merge",
+        }
+    },
+}
+
+
+FULL_EXTENSION_YAML = util.dump_yaml(FULL_EXTENSION_PROJECT)