diff --git a/src/core/server/http/router/response_adapter.ts b/src/core/server/http/router/response_adapter.ts
index fe6cb32d21776..e5dabc99f4442 100644
--- a/src/core/server/http/router/response_adapter.ts
+++ b/src/core/server/http/router/response_adapter.ts
@@ -120,7 +120,11 @@ export class HapiResponseAdapter {
     });
 
     error.output.payload.message = getErrorMessage(payload);
-    error.output.payload.attributes = getErrorAttributes(payload);
+
+    const attributes = getErrorAttributes(payload);
+    if (attributes) {
+      error.output.payload.attributes = attributes;
+    }
 
     const headers = kibanaResponse.options.headers;
     if (headers) {
diff --git a/src/core/server/ui_settings/routes/delete.ts b/src/core/server/ui_settings/routes/delete.ts
new file mode 100644
index 0000000000000..ee4c05325fcd4
--- /dev/null
+++ b/src/core/server/ui_settings/routes/delete.ts
@@ -0,0 +1,61 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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 { schema } from '@kbn/config-schema';
+
+import { IRouter } from '../../http';
+import { SavedObjectsErrorHelpers } from '../../saved_objects';
+import { CannotOverrideError } from '../ui_settings_errors';
+
+const validate = {
+  params: schema.object({
+    key: schema.string(),
+  }),
+};
+
+export function registerDeleteRoute(router: IRouter) {
+  router.delete(
+    { path: '/api/kibana/settings/{key}', validate },
+    async (context, request, response) => {
+      try {
+        const uiSettingsClient = context.core.uiSettings.client;
+
+        await uiSettingsClient.remove(request.params.key);
+
+        return response.ok({
+          body: {
+            settings: await uiSettingsClient.getUserProvided(),
+          },
+        });
+      } catch (error) {
+        if (SavedObjectsErrorHelpers.isSavedObjectsClientError(error)) {
+          return response.customError({
+            body: error,
+            statusCode: error.output.statusCode,
+          });
+        }
+
+        if (error instanceof CannotOverrideError) {
+          return response.badRequest({ body: error });
+        }
+
+        throw error;
+      }
+    }
+  );
+}
diff --git a/src/legacy/ui/ui_settings/routes/delete.ts b/src/core/server/ui_settings/routes/get.ts
similarity index 51%
rename from src/legacy/ui/ui_settings/routes/delete.ts
rename to src/core/server/ui_settings/routes/get.ts
index 7825204e6b99b..d249369a1ace7 100644
--- a/src/legacy/ui/ui_settings/routes/delete.ts
+++ b/src/core/server/ui_settings/routes/get.ts
@@ -16,22 +16,30 @@
  * specific language governing permissions and limitations
  * under the License.
  */
-import { Legacy } from 'kibana';
+import { IRouter } from '../../http';
+import { SavedObjectsErrorHelpers } from '../../saved_objects';
 
-async function handleRequest(request: Legacy.Request) {
-  const { key } = request.params;
-  const uiSettings = request.getUiSettingsService();
+export function registerGetRoute(router: IRouter) {
+  router.get(
+    { path: '/api/kibana/settings', validate: false },
+    async (context, request, response) => {
+      try {
+        const uiSettingsClient = context.core.uiSettings.client;
+        return response.ok({
+          body: {
+            settings: await uiSettingsClient.getUserProvided(),
+          },
+        });
+      } catch (error) {
+        if (SavedObjectsErrorHelpers.isSavedObjectsClientError(error)) {
+          return response.customError({
+            body: error,
+            statusCode: error.output.statusCode,
+          });
+        }
 
-  await uiSettings.remove(key);
-  return {
-    settings: await uiSettings.getUserProvided(),
-  };
+        throw error;
+      }
+    }
+  );
 }
-
-export const deleteRoute = {
-  path: '/api/kibana/settings/{key}',
-  method: 'DELETE',
-  handler: async (request: Legacy.Request) => {
-    return await handleRequest(request);
-  },
-};
diff --git a/src/legacy/ui/ui_settings/routes/get.ts b/src/core/server/ui_settings/routes/index.ts
similarity index 67%
rename from src/legacy/ui/ui_settings/routes/get.ts
rename to src/core/server/ui_settings/routes/index.ts
index 3e165a12522bb..d70d55d725938 100644
--- a/src/legacy/ui/ui_settings/routes/get.ts
+++ b/src/core/server/ui_settings/routes/index.ts
@@ -16,19 +16,16 @@
  * specific language governing permissions and limitations
  * under the License.
  */
-import { Legacy } from 'kibana';
+import { IRouter } from 'src/core/server';
 
-async function handleRequest(request: Legacy.Request) {
-  const uiSettings = request.getUiSettingsService();
-  return {
-    settings: await uiSettings.getUserProvided(),
-  };
-}
+import { registerDeleteRoute } from './delete';
+import { registerGetRoute } from './get';
+import { registerSetManyRoute } from './set_many';
+import { registerSetRoute } from './set';
 
-export const getRoute = {
-  path: '/api/kibana/settings',
-  method: 'GET',
-  handler(request: Legacy.Request) {
-    return handleRequest(request);
-  },
-};
+export function registerRoutes(router: IRouter) {
+  registerGetRoute(router);
+  registerDeleteRoute(router);
+  registerSetRoute(router);
+  registerSetManyRoute(router);
+}
diff --git a/src/legacy/ui/ui_settings/routes/integration_tests/doc_exists.ts b/src/core/server/ui_settings/routes/integration_tests/doc_exists.ts
similarity index 100%
rename from src/legacy/ui/ui_settings/routes/integration_tests/doc_exists.ts
rename to src/core/server/ui_settings/routes/integration_tests/doc_exists.ts
diff --git a/src/legacy/ui/ui_settings/routes/integration_tests/doc_missing.ts b/src/core/server/ui_settings/routes/integration_tests/doc_missing.ts
similarity index 100%
rename from src/legacy/ui/ui_settings/routes/integration_tests/doc_missing.ts
rename to src/core/server/ui_settings/routes/integration_tests/doc_missing.ts
diff --git a/src/legacy/ui/ui_settings/routes/integration_tests/doc_missing_and_index_read_only.ts b/src/core/server/ui_settings/routes/integration_tests/doc_missing_and_index_read_only.ts
similarity index 100%
rename from src/legacy/ui/ui_settings/routes/integration_tests/doc_missing_and_index_read_only.ts
rename to src/core/server/ui_settings/routes/integration_tests/doc_missing_and_index_read_only.ts
diff --git a/src/legacy/ui/ui_settings/routes/integration_tests/index.test.ts b/src/core/server/ui_settings/routes/integration_tests/index.test.ts
similarity index 100%
rename from src/legacy/ui/ui_settings/routes/integration_tests/index.test.ts
rename to src/core/server/ui_settings/routes/integration_tests/index.test.ts
diff --git a/src/legacy/ui/ui_settings/routes/integration_tests/lib/assert.ts b/src/core/server/ui_settings/routes/integration_tests/lib/assert.ts
similarity index 100%
rename from src/legacy/ui/ui_settings/routes/integration_tests/lib/assert.ts
rename to src/core/server/ui_settings/routes/integration_tests/lib/assert.ts
diff --git a/src/legacy/ui/ui_settings/routes/integration_tests/lib/chance.ts b/src/core/server/ui_settings/routes/integration_tests/lib/chance.ts
similarity index 100%
rename from src/legacy/ui/ui_settings/routes/integration_tests/lib/chance.ts
rename to src/core/server/ui_settings/routes/integration_tests/lib/chance.ts
diff --git a/src/legacy/ui/ui_settings/routes/integration_tests/lib/index.ts b/src/core/server/ui_settings/routes/integration_tests/lib/index.ts
similarity index 100%
rename from src/legacy/ui/ui_settings/routes/integration_tests/lib/index.ts
rename to src/core/server/ui_settings/routes/integration_tests/lib/index.ts
diff --git a/src/legacy/ui/ui_settings/routes/integration_tests/lib/servers.ts b/src/core/server/ui_settings/routes/integration_tests/lib/servers.ts
similarity index 97%
rename from src/legacy/ui/ui_settings/routes/integration_tests/lib/servers.ts
rename to src/core/server/ui_settings/routes/integration_tests/lib/servers.ts
index ae0ef1c91411e..cdcedc12b86e5 100644
--- a/src/legacy/ui/ui_settings/routes/integration_tests/lib/servers.ts
+++ b/src/core/server/ui_settings/routes/integration_tests/lib/servers.ts
@@ -20,7 +20,7 @@
 import { UnwrapPromise } from '@kbn/utility-types';
 import { SavedObjectsClientContract, IUiSettingsClient } from 'src/core/server';
 
-import KbnServer from '../../../../../server/kbn_server';
+import KbnServer from '../../../../../../legacy/server/kbn_server';
 import { createTestServers } from '../../../../../../test_utils/kbn_server';
 import { CallCluster } from '../../../../../../legacy/core_plugins/elasticsearch';
 
diff --git a/src/core/server/ui_settings/routes/set.ts b/src/core/server/ui_settings/routes/set.ts
new file mode 100644
index 0000000000000..51ad256b51335
--- /dev/null
+++ b/src/core/server/ui_settings/routes/set.ts
@@ -0,0 +1,67 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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 { schema } from '@kbn/config-schema';
+
+import { IRouter } from '../../http';
+import { SavedObjectsErrorHelpers } from '../../saved_objects';
+import { CannotOverrideError } from '../ui_settings_errors';
+
+const validate = {
+  params: schema.object({
+    key: schema.string(),
+  }),
+  body: schema.object({
+    value: schema.any(),
+  }),
+};
+
+export function registerSetRoute(router: IRouter) {
+  router.post(
+    { path: '/api/kibana/settings/{key}', validate },
+    async (context, request, response) => {
+      try {
+        const uiSettingsClient = context.core.uiSettings.client;
+
+        const { key } = request.params;
+        const { value } = request.body;
+
+        await uiSettingsClient.set(key, value);
+
+        return response.ok({
+          body: {
+            settings: await uiSettingsClient.getUserProvided(),
+          },
+        });
+      } catch (error) {
+        if (SavedObjectsErrorHelpers.isSavedObjectsClientError(error)) {
+          return response.customError({
+            body: error,
+            statusCode: error.output.statusCode,
+          });
+        }
+
+        if (error instanceof CannotOverrideError) {
+          return response.badRequest({ body: error });
+        }
+
+        throw error;
+      }
+    }
+  );
+}
diff --git a/src/core/server/ui_settings/routes/set_many.ts b/src/core/server/ui_settings/routes/set_many.ts
new file mode 100644
index 0000000000000..3794eba004bee
--- /dev/null
+++ b/src/core/server/ui_settings/routes/set_many.ts
@@ -0,0 +1,60 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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 { schema } from '@kbn/config-schema';
+
+import { IRouter } from '../../http';
+import { SavedObjectsErrorHelpers } from '../../saved_objects';
+import { CannotOverrideError } from '../ui_settings_errors';
+
+const validate = {
+  body: schema.object({
+    changes: schema.object({}, { allowUnknowns: true }),
+  }),
+};
+
+export function registerSetManyRoute(router: IRouter) {
+  router.post({ path: '/api/kibana/settings', validate }, async (context, request, response) => {
+    try {
+      const uiSettingsClient = context.core.uiSettings.client;
+
+      const { changes } = request.body;
+
+      await uiSettingsClient.setMany(changes);
+
+      return response.ok({
+        body: {
+          settings: await uiSettingsClient.getUserProvided(),
+        },
+      });
+    } catch (error) {
+      if (SavedObjectsErrorHelpers.isSavedObjectsClientError(error)) {
+        return response.customError({
+          body: error,
+          statusCode: error.output.statusCode,
+        });
+      }
+
+      if (error instanceof CannotOverrideError) {
+        return response.badRequest({ body: error });
+      }
+
+      throw error;
+    }
+  });
+}
diff --git a/src/core/server/ui_settings/ui_settings_client.test.ts b/src/core/server/ui_settings/ui_settings_client.test.ts
index 59c13fbebee70..addec464e1b09 100644
--- a/src/core/server/ui_settings/ui_settings_client.test.ts
+++ b/src/core/server/ui_settings/ui_settings_client.test.ts
@@ -24,6 +24,7 @@ import sinon from 'sinon';
 import { loggingServiceMock } from '../logging/logging_service.mock';
 
 import { UiSettingsClient } from './ui_settings_client';
+import { CannotOverrideError } from './ui_settings_errors';
 import * as createOrUpgradeSavedConfigNS from './create_or_upgrade_saved_config/create_or_upgrade_saved_config';
 import { createObjectsClientStub, savedObjectsClientErrors } from './create_objects_client_stub';
 
@@ -551,15 +552,14 @@ describe('ui settings', () => {
       const { uiSettings } = setup();
       expect(uiSettings.assertUpdateAllowed('foo')).to.be(undefined);
     });
-    it('throws 400 Boom error when keys is overridden', () => {
+    it('throws CannotOverrideError when key is overridden', () => {
       const { uiSettings } = setup({ overrides: { foo: true } });
       expect(() => uiSettings.assertUpdateAllowed('foo')).to.throwError(error => {
+        expect(error).to.be.a(CannotOverrideError);
         expect(error).to.have.property(
           'message',
           'Unable to update "foo" because it is overridden'
         );
-        expect(error).to.have.property('isBoom', true);
-        expect(error.output).to.have.property('statusCode', 400);
       });
     });
   });
diff --git a/src/core/server/ui_settings/ui_settings_client.ts b/src/core/server/ui_settings/ui_settings_client.ts
index 2be7a0801a533..013586695f3b1 100644
--- a/src/core/server/ui_settings/ui_settings_client.ts
+++ b/src/core/server/ui_settings/ui_settings_client.ts
@@ -17,12 +17,12 @@
  * under the License.
  */
 import { defaultsDeep } from 'lodash';
-import Boom from 'boom';
 
 import { SavedObjectsClientContract, SavedObjectAttribute } from '../saved_objects/types';
 import { Logger } from '../logging';
 import { createOrUpgradeSavedConfig } from './create_or_upgrade_saved_config';
 import { IUiSettingsClient, UiSettingsParams } from './types';
+import { CannotOverrideError } from './ui_settings_errors';
 
 export interface UiSettingsServiceOptions {
   type: string;
@@ -149,7 +149,7 @@ export class UiSettingsClient implements IUiSettingsClient {
   // NOTE: should be private method
   assertUpdateAllowed(key: string) {
     if (this.isOverridden(key)) {
-      throw Boom.badRequest(`Unable to update "${key}" because it is overridden`);
+      throw new CannotOverrideError(`Unable to update "${key}" because it is overridden`);
     }
   }
 
diff --git a/src/legacy/ui/ui_settings/routes/index.ts b/src/core/server/ui_settings/ui_settings_errors.ts
similarity index 66%
rename from src/legacy/ui/ui_settings/routes/index.ts
rename to src/core/server/ui_settings/ui_settings_errors.ts
index f3c9d4f0d8d14..d8fc32b111e44 100644
--- a/src/legacy/ui/ui_settings/routes/index.ts
+++ b/src/core/server/ui_settings/ui_settings_errors.ts
@@ -17,7 +17,15 @@
  * under the License.
  */
 
-export { deleteRoute } from './delete';
-export { getRoute } from './get';
-export { setManyRoute } from './set_many';
-export { setRoute } from './set';
+export class CannotOverrideError extends Error {
+  public cause?: Error;
+
+  constructor(message: string, cause?: Error) {
+    super(message);
+    this.cause = cause;
+
+    // Set the prototype explicitly, see:
+    // https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#extending-built-ins-like-error-array-and-map-may-no-longer-work
+    Object.setPrototypeOf(this, CannotOverrideError.prototype);
+  }
+}
diff --git a/src/core/server/ui_settings/ui_settings_service.ts b/src/core/server/ui_settings/ui_settings_service.ts
index c45f04837d26e..4c19386fcba7a 100644
--- a/src/core/server/ui_settings/ui_settings_service.ts
+++ b/src/core/server/ui_settings/ui_settings_service.ts
@@ -30,6 +30,8 @@ import { UiSettingsClient } from './ui_settings_client';
 import { InternalUiSettingsServiceSetup, UiSettingsParams } from './types';
 import { mapToObject } from '../../utils/';
 
+import { registerRoutes } from './routes';
+
 interface SetupDeps {
   http: InternalHttpServiceSetup;
 }
@@ -46,6 +48,7 @@ export class UiSettingsService implements CoreService<InternalUiSettingsServiceS
   }
 
   public async setup(deps: SetupDeps): Promise<InternalUiSettingsServiceSetup> {
+    registerRoutes(deps.http.createRouter(''));
     this.log.debug('Setting up ui settings service');
     const overrides = await this.getOverrides(deps);
     const { version, buildNum } = this.coreContext.env.packageInfo;
diff --git a/src/legacy/ui/ui_settings/routes/set.ts b/src/legacy/ui/ui_settings/routes/set.ts
deleted file mode 100644
index 1f1ab17a0daf7..0000000000000
--- a/src/legacy/ui/ui_settings/routes/set.ts
+++ /dev/null
@@ -1,55 +0,0 @@
-/*
- * Licensed to Elasticsearch B.V. under one or more contributor
- * license agreements. See the NOTICE file distributed with
- * this work for additional information regarding copyright
- * ownership. Elasticsearch B.V. licenses this file to you 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 { Legacy } from 'kibana';
-import Joi from 'joi';
-
-async function handleRequest(request: Legacy.Request) {
-  const { key } = request.params;
-  const { value } = request.payload as any;
-  const uiSettings = request.getUiSettingsService();
-
-  await uiSettings.set(key, value);
-
-  return {
-    settings: await uiSettings.getUserProvided(),
-  };
-}
-
-export const setRoute = {
-  path: '/api/kibana/settings/{key}',
-  method: 'POST',
-  config: {
-    validate: {
-      params: Joi.object()
-        .keys({
-          key: Joi.string().required(),
-        })
-        .default(),
-
-      payload: Joi.object()
-        .keys({
-          value: Joi.any().required(),
-        })
-        .required(),
-    },
-    handler(request: Legacy.Request) {
-      return handleRequest(request);
-    },
-  },
-};
diff --git a/src/legacy/ui/ui_settings/routes/set_many.ts b/src/legacy/ui/ui_settings/routes/set_many.ts
deleted file mode 100644
index 18b1046417fec..0000000000000
--- a/src/legacy/ui/ui_settings/routes/set_many.ts
+++ /dev/null
@@ -1,50 +0,0 @@
-/*
- * Licensed to Elasticsearch B.V. under one or more contributor
- * license agreements. See the NOTICE file distributed with
- * this work for additional information regarding copyright
- * ownership. Elasticsearch B.V. licenses this file to you 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 { Legacy } from 'kibana';
-import Joi from 'joi';
-
-async function handleRequest(request: Legacy.Request) {
-  const { changes } = request.payload as any;
-  const uiSettings = request.getUiSettingsService();
-
-  await uiSettings.setMany(changes);
-
-  return {
-    settings: await uiSettings.getUserProvided(),
-  };
-}
-
-export const setManyRoute = {
-  path: '/api/kibana/settings',
-  method: 'POST',
-  config: {
-    validate: {
-      payload: Joi.object()
-        .keys({
-          changes: Joi.object()
-            .unknown(true)
-            .required(),
-        })
-        .required(),
-    },
-    handler(request: Legacy.Request) {
-      return handleRequest(request);
-    },
-  },
-};
diff --git a/src/legacy/ui/ui_settings/ui_settings_mixin.js b/src/legacy/ui/ui_settings/ui_settings_mixin.js
index 8c7ef25c6f8d7..ae9cde541b76f 100644
--- a/src/legacy/ui/ui_settings/ui_settings_mixin.js
+++ b/src/legacy/ui/ui_settings/ui_settings_mixin.js
@@ -19,12 +19,6 @@
 
 import { uiSettingsServiceFactory } from './ui_settings_service_factory';
 import { getUiSettingsServiceForRequest } from './ui_settings_service_for_request';
-import {
-  deleteRoute,
-  getRoute,
-  setManyRoute,
-  setRoute,
-} from './routes';
 
 export function uiSettingsMixin(kbnServer, server) {
   const { uiSettingDefaults = {} } = kbnServer.uiExports;
@@ -58,9 +52,4 @@ export function uiSettingsMixin(kbnServer, server) {
       server.uiSettings has been removed, see https://github.com/elastic/kibana/pull/12243.
     `);
   });
-
-  server.route(deleteRoute);
-  server.route(getRoute);
-  server.route(setManyRoute);
-  server.route(setRoute);
 }