diff --git a/.changeset/tidy-camels-boil.md b/.changeset/tidy-camels-boil.md
new file mode 100644
index 0000000000..6e59f96f34
--- /dev/null
+++ b/.changeset/tidy-camels-boil.md
@@ -0,0 +1,5 @@
+---
+'@backstage/core-app-api': patch
+---
+
+Add component data `core.type` marker for `AppRouter` and `FlatRoutes`.
diff --git a/packages/core-app-api/src/app/AppRouter.tsx b/packages/core-app-api/src/app/AppRouter.tsx
index 9d07be1ffd..7481f6662a 100644
--- a/packages/core-app-api/src/app/AppRouter.tsx
+++ b/packages/core-app-api/src/app/AppRouter.tsx
@@ -16,6 +16,7 @@
 
 import React, { useContext, ReactNode, ComponentType, useState } from 'react';
 import {
+  attachComponentData,
   ConfigApi,
   configApiRef,
   IdentityApi,
@@ -186,3 +187,5 @@ export function AppRouter(props: AppRouterProps) {
     </RouterComponent>
   );
 }
+
+attachComponentData(AppRouter, 'core.type', 'AppRouter');
diff --git a/packages/core-app-api/src/routing/FlatRoutes.tsx b/packages/core-app-api/src/routing/FlatRoutes.tsx
index abb7fe9432..46c2270d0d 100644
--- a/packages/core-app-api/src/routing/FlatRoutes.tsx
+++ b/packages/core-app-api/src/routing/FlatRoutes.tsx
@@ -16,7 +16,11 @@
 
 import React, { ReactNode, useMemo } from 'react';
 import { useRoutes } from 'react-router-dom';
-import { useApp, useElementFilter } from '@backstage/core-plugin-api';
+import {
+  attachComponentData,
+  useApp,
+  useElementFilter,
+} from '@backstage/core-plugin-api';
 import { isReactRouterBeta } from '../app/isReactRouterBeta';
 
 let warned = false;
@@ -115,3 +119,5 @@ export const FlatRoutes = (props: FlatRoutesProps): JSX.Element | null => {
 
   return useRoutes(withNotFound);
 };
+
+attachComponentData(FlatRoutes, 'core.type', 'FlatRoutes');
diff --git a/packages/core-compat-api/api-report.md b/packages/core-compat-api/api-report.md
index 5175c76ac8..3dd2eb6b22 100644
--- a/packages/core-compat-api/api-report.md
+++ b/packages/core-compat-api/api-report.md
@@ -6,11 +6,18 @@
 /// <reference types="react" />
 
 import { BackstagePlugin } from '@backstage/frontend-plugin-api';
+import { ExtensionOverrides } from '@backstage/frontend-plugin-api';
+import { default as React_2 } from 'react';
 
 // @public (undocumented)
 export function collectLegacyRoutes(
   flatRoutesElement: JSX.Element,
 ): BackstagePlugin[];
 
+// @public (undocumented)
+export function convertLegacyApp(
+  rootElement: React_2.JSX.Element,
+): (ExtensionOverrides | BackstagePlugin)[];
+
 // (No @packageDocumentation comment for this package)
 ```
diff --git a/packages/core-compat-api/src/collectLegacyRoutes.test.tsx b/packages/core-compat-api/src/collectLegacyRoutes.test.tsx
index 486c373991..85fed83e8c 100644
--- a/packages/core-compat-api/src/collectLegacyRoutes.test.tsx
+++ b/packages/core-compat-api/src/collectLegacyRoutes.test.tsx
@@ -30,6 +30,7 @@ describe('collectLegacyRoutes', () => {
         <Route path="/score-board" element={<ScoreBoardPage />} />
         <Route path="/stackstorm" element={<StackstormPage />} />
         <Route path="/puppetdb" element={<PuppetDbPage />} />
+        <Route path="/puppetdb" element={<PuppetDbPage />} />
       </FlatRoutes>,
     );
 
@@ -85,6 +86,12 @@ describe('collectLegacyRoutes', () => {
             disabled: false,
             defaultConfig: { path: 'puppetdb' },
           },
+          {
+            id: 'plugin.puppetDb.page2',
+            attachTo: { id: 'core.routes', input: 'routes' },
+            disabled: false,
+            defaultConfig: { path: 'puppetdb' },
+          },
           {
             id: 'apis.plugin.puppetdb.service',
             attachTo: { id: 'core', input: 'apis' },
diff --git a/packages/core-compat-api/src/collectLegacyRoutes.tsx b/packages/core-compat-api/src/collectLegacyRoutes.tsx
index 2b149e6285..8178de2876 100644
--- a/packages/core-compat-api/src/collectLegacyRoutes.tsx
+++ b/packages/core-compat-api/src/collectLegacyRoutes.tsx
@@ -62,7 +62,10 @@ Existing tasks:
 export function collectLegacyRoutes(
   flatRoutesElement: JSX.Element,
 ): BackstagePlugin[] {
-  const results = new Array<BackstagePlugin>();
+  const createdPluginIds = new Map<
+    LegacyBackstagePlugin,
+    Extension<unknown>[]
+  >();
 
   React.Children.forEach(
     flatRoutesElement.props.children,
@@ -93,13 +96,18 @@ export function collectLegacyRoutes(
       );
 
       const pluginId = plugin.getId();
-      const path: string = route.props.path;
 
-      const detectedExtensions = new Array<Extension<unknown>>();
+      const detectedExtensions =
+        createdPluginIds.get(plugin) ?? new Array<Extension<unknown>>();
+      createdPluginIds.set(plugin, detectedExtensions);
+
+      const path: string = route.props.path;
 
       detectedExtensions.push(
         createPageExtension({
-          id: `plugin.${pluginId}.page`,
+          id: `plugin.${pluginId}.page${
+            detectedExtensions.length ? detectedExtensions.length + 1 : ''
+          }`,
           defaultPath: path[0] === '/' ? path.slice(1) : path,
           routeRef: routeRef ? convertLegacyRouteRef(routeRef) : undefined,
 
@@ -115,23 +123,20 @@ export function collectLegacyRoutes(
             ),
         }),
       );
+    },
+  );
 
-      detectedExtensions.push(
+  return Array.from(createdPluginIds).map(([plugin, extensions]) =>
+    createPlugin({
+      id: plugin.getId(),
+      extensions: [
+        ...extensions,
         ...Array.from(plugin.getApis()).map(factory =>
           createApiExtension({
             factory,
           }),
         ),
-      );
-
-      results.push(
-        createPlugin({
-          id: plugin.getId(),
-          extensions: detectedExtensions,
-        }),
-      );
-    },
+      ],
+    }),
   );
-
-  return results;
 }
diff --git a/packages/core-compat-api/src/convertLegacyApp.test.tsx b/packages/core-compat-api/src/convertLegacyApp.test.tsx
new file mode 100644
index 0000000000..0f1f6145c9
--- /dev/null
+++ b/packages/core-compat-api/src/convertLegacyApp.test.tsx
@@ -0,0 +1,129 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { AppRouter, FlatRoutes } from '@backstage/core-app-api';
+import { PuppetDbPage } from '@backstage/plugin-puppetdb';
+import { StackstormPage } from '@backstage/plugin-stackstorm';
+import { ScoreBoardPage } from '@oriflame/backstage-plugin-score-card';
+import React, { ReactNode } from 'react';
+import { Route } from 'react-router-dom';
+import { convertLegacyApp } from './convertLegacyApp';
+
+const Root = ({ children }: { children: ReactNode }) => <>{children}</>;
+
+describe('convertLegacyApp', () => {
+  it('should find and extract root and routes', () => {
+    const collected = convertLegacyApp(
+      <>
+        <div />
+        <span />
+        <AppRouter>
+          <div />
+          <Root>
+            <FlatRoutes>
+              <Route path="/score-board" element={<ScoreBoardPage />} />
+              <Route path="/stackstorm" element={<StackstormPage />} />
+              <Route path="/puppetdb" element={<PuppetDbPage />} />
+              <Route path="/puppetdb" element={<PuppetDbPage />} />
+            </FlatRoutes>
+          </Root>
+        </AppRouter>
+      </>,
+    );
+
+    expect(
+      collected.map((p: any /* TODO */) => ({
+        id: p.id,
+        extensions: p.extensions.map((e: any) => ({
+          id: e.id,
+          attachTo: e.attachTo,
+          disabled: e.disabled,
+          defaultConfig: e.configSchema?.parse({}),
+        })),
+      })),
+    ).toEqual([
+      {
+        id: 'score-card',
+        extensions: [
+          {
+            id: 'plugin.score-card.page',
+            attachTo: { id: 'core.routes', input: 'routes' },
+            disabled: false,
+            defaultConfig: { path: 'score-board' },
+          },
+          {
+            id: 'apis.plugin.scoringdata.service',
+            attachTo: { id: 'core', input: 'apis' },
+            disabled: false,
+          },
+        ],
+      },
+      {
+        id: 'stackstorm',
+        extensions: [
+          {
+            id: 'plugin.stackstorm.page',
+            attachTo: { id: 'core.routes', input: 'routes' },
+            disabled: false,
+            defaultConfig: { path: 'stackstorm' },
+          },
+          {
+            id: 'apis.plugin.stackstorm.service',
+            attachTo: { id: 'core', input: 'apis' },
+            disabled: false,
+          },
+        ],
+      },
+      {
+        id: 'puppetDb',
+        extensions: [
+          {
+            id: 'plugin.puppetDb.page',
+            attachTo: { id: 'core.routes', input: 'routes' },
+            disabled: false,
+            defaultConfig: { path: 'puppetdb' },
+          },
+          {
+            id: 'plugin.puppetDb.page2',
+            attachTo: { id: 'core.routes', input: 'routes' },
+            disabled: false,
+            defaultConfig: { path: 'puppetdb' },
+          },
+          {
+            id: 'apis.plugin.puppetdb.service',
+            attachTo: { id: 'core', input: 'apis' },
+            disabled: false,
+          },
+        ],
+      },
+      {
+        id: undefined,
+        extensions: [
+          {
+            id: 'core.layout',
+            attachTo: { id: 'core', input: 'root' },
+            disabled: false,
+          },
+          {
+            id: 'core.nav',
+            attachTo: { id: 'core.layout', input: 'nav' },
+            disabled: true,
+          },
+        ],
+      },
+    ]);
+  });
+});
diff --git a/packages/core-compat-api/src/convertLegacyApp.ts b/packages/core-compat-api/src/convertLegacyApp.ts
new file mode 100644
index 0000000000..59cb136c65
--- /dev/null
+++ b/packages/core-compat-api/src/convertLegacyApp.ts
@@ -0,0 +1,139 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import React, {
+  Children,
+  Fragment,
+  ReactElement,
+  ReactNode,
+  isValidElement,
+} from 'react';
+import {
+  BackstagePlugin,
+  ExtensionOverrides,
+  coreExtensionData,
+  createExtension,
+  createExtensionInput,
+  createExtensionOverrides,
+} from '@backstage/frontend-plugin-api';
+import { getComponentData } from '@backstage/core-plugin-api';
+import { collectLegacyRoutes } from './collectLegacyRoutes';
+
+function selectChildren(
+  rootNode: ReactNode,
+  selector?: (element: ReactElement<{ children?: ReactNode }>) => boolean,
+  strictError?: string,
+): Array<ReactElement<{ children?: ReactNode }>> {
+  return Children.toArray(rootNode).flatMap(node => {
+    if (!isValidElement<{ children?: ReactNode }>(node)) {
+      return [];
+    }
+
+    if (node.type === Fragment) {
+      return selectChildren(node.props.children, selector, strictError);
+    }
+
+    if (selector === undefined || selector(node)) {
+      return [node];
+    }
+
+    if (strictError) {
+      throw new Error(strictError);
+    }
+
+    return selectChildren(node.props.children, selector, strictError);
+  });
+}
+
+/** @public */
+export function convertLegacyApp(
+  rootElement: React.JSX.Element,
+): (ExtensionOverrides | BackstagePlugin)[] {
+  const appRouterEls = selectChildren(
+    rootElement,
+    el => getComponentData(el, 'core.type') === 'AppRouter',
+  );
+  if (appRouterEls.length !== 1) {
+    throw new Error(
+      "Failed to convert legacy app, AppRouter element could not been found. Make sure it's at the top level of the App element tree",
+    );
+  }
+
+  const rootEls = selectChildren(
+    appRouterEls[0].props.children,
+    el =>
+      Boolean(el.props.children) &&
+      selectChildren(
+        el.props.children,
+        innerEl => getComponentData(innerEl, 'core.type') === 'FlatRoutes',
+      ).length === 1,
+  );
+  if (rootEls.length !== 1) {
+    throw new Error(
+      "Failed to convert legacy app, Root element containing FlatRoutes could not been found. Make sure it's within the AppRouter element of the App element tree",
+    );
+  }
+  const [rootEl] = rootEls;
+
+  const routesEls = selectChildren(
+    rootEls[0].props.children,
+    el => getComponentData(el, 'core.type') === 'FlatRoutes',
+  );
+  if (routesEls.length !== 1) {
+    throw new Error(
+      'Unexpectedly failed to find FlatRoutes in app element tree',
+    );
+  }
+  const [routesEl] = routesEls;
+
+  const CoreLayoutOverride = createExtension({
+    id: 'core.layout',
+    attachTo: { id: 'core', input: 'root' },
+    inputs: {
+      content: createExtensionInput(
+        {
+          element: coreExtensionData.reactElement,
+        },
+        { singleton: true },
+      ),
+    },
+    output: {
+      element: coreExtensionData.reactElement,
+    },
+    factory({ bind, inputs }) {
+      // Clone the root element, this replaces the FlatRoutes declared in the app with out content input
+      bind({
+        element: React.cloneElement(rootEl, undefined, inputs.content.element),
+      });
+    },
+  });
+  const CoreNavOverride = createExtension({
+    id: 'core.nav',
+    attachTo: { id: 'core.layout', input: 'nav' },
+    output: {},
+    factory() {},
+    disabled: true,
+  });
+
+  const collectedRoutes = collectLegacyRoutes(routesEl);
+
+  return [
+    ...collectedRoutes,
+    createExtensionOverrides({
+      extensions: [CoreLayoutOverride, CoreNavOverride],
+    }),
+  ];
+}
diff --git a/packages/core-compat-api/src/index.ts b/packages/core-compat-api/src/index.ts
index 2181b2fa9a..e5f61119a3 100644
--- a/packages/core-compat-api/src/index.ts
+++ b/packages/core-compat-api/src/index.ts
@@ -14,3 +14,4 @@
  * limitations under the License.
  */
 export { collectLegacyRoutes } from './collectLegacyRoutes';
+export { convertLegacyApp } from './convertLegacyApp';
diff --git a/packages/frontend-app-api/src/wiring/createExtensionInstance.ts b/packages/frontend-app-api/src/wiring/createExtensionInstance.ts
index 5a325dc0a0..767329da7f 100644
--- a/packages/frontend-app-api/src/wiring/createExtensionInstance.ts
+++ b/packages/frontend-app-api/src/wiring/createExtensionInstance.ts
@@ -63,6 +63,7 @@ function resolveInputs(
   const undeclaredAttachments = Array.from(attachments.entries()).filter(
     ([inputName]) => inputMap[inputName] === undefined,
   );
+  // TODO: Make this a warning rather than an error
   if (undeclaredAttachments.length > 0) {
     throw new Error(
       `received undeclared input${