diff --git a/rockcraft/extensions/express.py b/rockcraft/extensions/express.py
index aeef053f..db493e7c 100644
--- a/rockcraft/extensions/express.py
+++ b/rockcraft/extensions/express.py
@@ -15,33 +15,43 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 """An extension for the NodeJS based Javascript application extension."""
+import json
+import re
 from typing import Any, Dict, Tuple
 
 from overrides import override
 
+from ..errors import ExtensionError
 from .extension import Extension
 
 
 class ExpressJSFramework(Extension):
     """An extension for constructing Javascript applications based on the ExpressJS framework."""
 
+    IMAGE_BASE_DIR = "app"
+    EXPRESS_GENERATOR_DIRS = (
+        "bin",
+        "public",
+        "routes",
+        "views",
+        "app.js",
+        "package.json",
+        "package-lock.json",
+        "node_modules",
+    )
+    RUNTIME_DEPENDENCIES = ["ca-certificates_data", "libpq5", "node"]
+
     @staticmethod
     @override
     def get_supported_bases() -> Tuple[str, ...]:
         """Return supported bases."""
-        return "bare", "ubuntu@22.04", "ubuntu:22.04", "ubuntu@24.04", "ubuntu:24.04"
+        return "bare", "ubuntu@22.04", "ubuntu@24.04"
 
     @staticmethod
     @override
     def is_experimental(base: str | None) -> bool:
         """Check if the extension is in an experimental state."""
-        return False
-
-    @property
-    @override
-    def framework(self) -> str:
-        """Return the framework name, i.e. expressjs."""
-        return "expressjs"
+        return True
 
     @override
     def get_root_snippet(self) -> Dict[str, Any]:
@@ -54,6 +64,8 @@ def get_root_snippet(self) -> Dict[str, Any]:
           - services: a service to run the ExpressJS server
           - parts: see ExpressJSFramework._gen_parts
         """
+        self._check_project()
+
         snippet: Dict[str, Any] = {
             "run-user": "_daemon_",
             "services": {
@@ -63,10 +75,15 @@ def get_root_snippet(self) -> Dict[str, Any]:
                     "startup": "enabled",
                     "on-success": "shutdown",
                     "on-failure": "shutdown",
-                    "working-dir": f"{self.framework}/app",
+                    "working-dir": "/app",
                 }
             },
         }
+
+        snippet["parts"] = {
+            "expressjs-framework/install-app": self._gen_install_app_part(),
+            "expressjs-framework/runtime-dependencies": self._gen_runtime_dependencies_part(),
+        }
         return snippet
 
     @override
@@ -85,37 +102,86 @@ def get_parts_snippet(self) -> dict[str, Any]:
         """
         return {}
 
-    def _gen_parts(self) -> dict:
-        """Generate the parts associated with this extension.
-
-        The parts generated are the following:
-        1. install-app: Install the application with npm installs.
-        2. install-dependencies: Install application host dependencies (e.g. pg lib)
-        """
-        ...
-
     def _gen_install_app_part(self) -> dict:
         """Generate the install app part using NPM plugin."""
-        ...
+        return {
+            "plugin": "npm",
+            "npm-include-node": False,
+            "source": "app/",
+            "organise": self._app_organise,
+            "override-prime": f"rm -rf lib/node_modules/{self._app_name}",
+        }
 
-    def _gen_install_dependencies_part(self) -> dict:
+    def _gen_runtime_dependencies_part(self) -> dict:
         """Generate the install dependencies part using dump plugin."""
-        ...
+        return {
+            "plugin": "nil",
+            "stage-packages": self.RUNTIME_DEPENDENCIES,
+        }
+
+    @property
+    def _app_package_json(self):
+        """Return the app package.json contents."""
+        package_json_file = self.project_root / "package.json"
+        if not package_json_file.exists():
+            raise ExtensionError(
+                "missing package.json file",
+                doc_slug="/reference/extensions/expressjs-framework",
+                logpath_report=False,
+            )
+        package_json_contents = package_json_file.read_text(encoding="utf-8")
+        return json.loads(package_json_contents)
+
+    @property
+    def _app_name(self) -> str:
+        """Return the application name as defined on package.json."""
+        return self._app_package_json["name"]
 
     @property
-    def _app_prime(self):
-        """Return the prime list for the ExpressJS proejct.
+    def _app_organise(self):
+        """Return the organised mapping for the ExpressJS project.
 
         Use the paths generated by the
         express-generator (https://expressjs.com/en/starter/generator.html) tool by default if no
         user prime paths are provided. Use only user prime paths otherwise.
         """
-        ...
+        user_prime: list[str] = (
+            self.yaml_data.get("parts", {})
+            .get("expressjs-framework/install-app", {})
+            .get("prime", [])
+        )
+        if not all(re.match(f"-? *{self.IMAGE_BASE_DIR}/", p) for p in user_prime):
+            raise ExtensionError(
+                "expressjs-framework extension requires the 'prime' entry in the "
+                f"expressjs-framework/install-app part to start with {self.IMAGE_BASE_DIR}/",
+                doc_slug="/reference/extensions/expressjs-framework",
+                logpath_report=False,
+            )
+        if not user_prime:
+            user_prime = [
+                f"{self.project_root}/{f}" for f in self.EXPRESS_GENERATOR_DIRS
+            ]
+        lib_dir = f"lib/node_modules/{self._app_name}"
+        return {
+            f"{lib_dir}/{f}": f"app/{f}"
+            for f in user_prime
+            if (self.project_root / f).exists()
+        }
 
     def _check_project(self):
         """Ensure this extension can apply to the current rockcraft project.
 
         The ExpressJS framework assumes that:
         - The npm start script exists.
+        - The application name is defined.
         """
-        ...
+        if (
+            "scripts" not in self._app_package_json
+            or "start" not in self._app_package_json["scripts"]
+            or "name" not in self._app_package_json
+        ):
+            raise ExtensionError(
+                "missing start script",
+                doc_slug="/reference/extensions/expressjs-framework",
+                logpath_report=False,
+            )