diff --git a/src/api/build.ts b/src/api/build.ts
index fbfdf5a4b..6038dbc15 100644
--- a/src/api/build.ts
+++ b/src/api/build.ts
@@ -19,6 +19,7 @@ type Opts = {
 	static?: string;
 	legacy?: boolean;
 	bundler?: 'rollup' | 'webpack';
+	ext?: string;
 	oncompile?: ({ type, result }: { type: string, result: CompileResult }) => void;
 };
 
@@ -32,6 +33,7 @@ export async function build({
 
 	bundler,
 	legacy = false,
+	ext,
 	oncompile = noop
 }: Opts = {}) {
 	bundler = validate_bundler(bundler);
@@ -68,7 +70,7 @@ export async function build({
 
 	fs.writeFileSync(`${dest}/template.html`, minify_html(template));
 
-	const manifest_data = create_manifest_data(routes);
+	const manifest_data = create_manifest_data(routes, ext);
 
 	// create src/node_modules/@sapper/app.mjs and server.mjs
 	create_app({
diff --git a/src/api/dev.ts b/src/api/dev.ts
index adfd73a92..c7e2575ce 100644
--- a/src/api/dev.ts
+++ b/src/api/dev.ts
@@ -28,7 +28,8 @@ type Opts = {
 	hot?: boolean,
 	'devtools-port'?: number,
 	bundler?: 'rollup' | 'webpack',
-	port?: number
+	port?: number,
+	ext: string
 };
 
 export function dev(opts: Opts) {
@@ -47,7 +48,7 @@ class Watcher extends EventEmitter {
 	}
 	port: number;
 	closed: boolean;
-
+	
 	dev_port: number;
 	live: boolean;
 	hot: boolean;
@@ -67,6 +68,7 @@ class Watcher extends EventEmitter {
 		unique_warnings: Set<string>;
 		unique_errors: Set<string>;
 	}
+	ext: string;
 
 	constructor({
 		cwd = '.',
@@ -80,7 +82,8 @@ class Watcher extends EventEmitter {
 		hot,
 		'devtools-port': devtools_port,
 		bundler,
-		port = +process.env.PORT
+		port = +process.env.PORT, 
+		ext
 	}: Opts) {
 		super();
 
@@ -95,7 +98,7 @@ class Watcher extends EventEmitter {
 			output: path.resolve(cwd, output),
 			static: path.resolve(cwd, static_files)
 		};
-
+		this.ext = ext;
 		this.port = port;
 		this.closed = false;
 
@@ -161,7 +164,7 @@ class Watcher extends EventEmitter {
 		let manifest_data: ManifestData;
 
 		try {
-			manifest_data = create_manifest_data(routes);
+			manifest_data = create_manifest_data(routes, this.ext);
 			create_app({
 				bundler: this.bundler,
 				manifest_data,
@@ -189,7 +192,7 @@ class Watcher extends EventEmitter {
 				},
 				() => {
 					try {
-						const new_manifest_data = create_manifest_data(routes);
+						const new_manifest_data = create_manifest_data(routes, this.ext);
 						create_app({
 							bundler: this.bundler,
 							manifest_data, // TODO is this right? not new_manifest_data?
diff --git a/src/cli.ts b/src/cli.ts
index 5ebacc96f..72024f35d 100755
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -31,6 +31,7 @@ prog.command('dev')
 	.option('--static', 'Static files directory', 'static')
 	.option('--output', 'Sapper output directory', 'src/node_modules/@sapper')
 	.option('--build-dir', 'Development build directory', '__sapper__/dev')
+	.option('--ext', 'Custom Route Extension', '.svelte .html')
 	.action(async (opts: {
 		port: number,
 		open: boolean,
@@ -43,7 +44,8 @@ prog.command('dev')
 		routes: string,
 		static: string,
 		output: string,
-		'build-dir': string
+		'build-dir': string,
+		ext: string
 	}) => {
 		const { dev } = await import('./api/dev');
 
@@ -59,7 +61,8 @@ prog.command('dev')
 				'dev-port': opts['dev-port'],
 				live: opts.live,
 				hot: opts.hot,
-				bundler: opts.bundler
+				bundler: opts.bundler,
+				ext: opts.ext
 			});
 
 			let first = true;
@@ -151,6 +154,7 @@ prog.command('build [dest]')
 	.option('--src', 'Source directory', 'src')
 	.option('--routes', 'Routes directory', 'src/routes')
 	.option('--output', 'Sapper output directory', 'src/node_modules/@sapper')
+	.option('--ext', 'Custom Route Extension', '.svelte .html')
 	.example(`build custom-dir -p 4567`)
 	.action(async (dest = '__sapper__/build', opts: {
 		port: string,
@@ -159,12 +163,13 @@ prog.command('build [dest]')
 		cwd: string,
 		src: string,
 		routes: string,
-		output: string
+		output: string,
+		ext: string
 	}) => {
 		console.log(`> Building...`);
 
 		try {
-			await _build(opts.bundler, opts.legacy, opts.cwd, opts.src, opts.routes, opts.output, dest);
+			await _build(opts.bundler, opts.legacy, opts.cwd, opts.src, opts.routes, opts.output, dest, opts.ext);
 
 			const launcher = path.resolve(dest, 'index.js');
 
@@ -198,6 +203,7 @@ prog.command('export [dest]')
 	.option('--static', 'Static files directory', 'static')
 	.option('--output', 'Sapper output directory', 'src/node_modules/@sapper')
 	.option('--build-dir', 'Intermediate build directory', '__sapper__/build')
+	.option('--ext', 'Custom Route Extension', '.svelte .html')
 	.action(async (dest = '__sapper__/export', opts: {
 		build: boolean,
 		legacy: boolean,
@@ -210,11 +216,12 @@ prog.command('export [dest]')
 		static: string,
 		output: string,
 		'build-dir': string,
+		ext: string
 	}) => {
 		try {
 			if (opts.build) {
 				console.log(`> Building...`);
-				await _build(opts.bundler, opts.legacy, opts.cwd, opts.src, opts.routes, opts.output, opts['build-dir']);
+				await _build(opts.bundler, opts.legacy, opts.cwd, opts.src, opts.routes, opts.output, opts['build-dir'], opts.ext);
 				console.error(`\n> Built in ${elapsed(start)}`);
 			}
 
@@ -262,7 +269,8 @@ async function _build(
 	src: string,
 	routes: string,
 	output: string,
-	dest: string
+	dest: string,
+	ext: string
 ) {
 	const { build } = await import('./api/build');
 
@@ -273,7 +281,7 @@ async function _build(
 		src,
 		routes,
 		dest,
-
+		ext,
 		oncompile: event => {
 			let banner = `built ${event.type}`;
 			let c = (txt: string) => colors.cyan(txt);
diff --git a/src/core/create_manifest_data.ts b/src/core/create_manifest_data.ts
index 6324733ed..4e4f2c6b2 100644
--- a/src/core/create_manifest_data.ts
+++ b/src/core/create_manifest_data.ts
@@ -4,9 +4,10 @@ import svelte from 'svelte/compiler';
 import { Page, PageComponent, ServerRoute, ManifestData } from '../interfaces';
 import { posixify, reserved_words } from '../utils';
 
-const component_extensions = ['.svelte', '.html']; // TODO make this configurable (to include e.g. .svelte.md?)
+export default function create_manifest_data(cwd: string, extensions: string = '.svelte .html'): ManifestData {
+	
+	const component_extensions = extensions.split(' ');
 
-export default function create_manifest_data(cwd: string): ManifestData {
 	// TODO remove in a future version
 	if (!fs.existsSync(cwd)) {
 		throw new Error(`As of Sapper 0.21, the routes/ directory should become src/routes/`);
diff --git a/test/apps/custom-extension/rollup.config.js b/test/apps/custom-extension/rollup.config.js
new file mode 100644
index 000000000..7a8516673
--- /dev/null
+++ b/test/apps/custom-extension/rollup.config.js
@@ -0,0 +1,60 @@
+import resolve from 'rollup-plugin-node-resolve';
+import replace from 'rollup-plugin-replace';
+import svelte from 'rollup-plugin-svelte';
+
+const mode = process.env.NODE_ENV;
+const dev = mode === 'development';
+
+const config = require('../../../config/rollup.js');
+
+export default {
+	client: {
+		input: config.client.input(),
+		output: config.client.output(),
+		plugins: [
+			replace({
+				'process.browser': true,
+				'process.env.NODE_ENV': JSON.stringify(mode)
+			}),
+			svelte({
+				extensions: ['.whokilledthemuffinman', '.jesuslivesineveryone', '.mdx', '.svelte', '.html'],
+				dev,
+				hydratable: true,
+				emitCss: true
+			}),
+			resolve()
+		]
+	},
+
+	server: {
+		input: config.server.input(),
+		output: config.server.output(),
+		plugins: [
+			replace({
+				'process.browser': false,
+				'process.env.NODE_ENV': JSON.stringify(mode)
+			}),
+			svelte({
+				extensions: ['.whokilledthemuffinman', '.jesuslivesineveryone', '.mdx', '.svelte', '.html'],
+				generate: 'ssr',
+				dev
+			}),
+			resolve({
+				preferBuiltins: true
+			})
+		],
+		external: ['sirv', 'polka']
+	},
+
+	serviceworker: {
+		input: config.serviceworker.input(),
+		output: config.serviceworker.output(),
+		plugins: [
+			resolve(),
+			replace({
+				'process.browser': true,
+				'process.env.NODE_ENV': JSON.stringify(mode)
+			})
+		]
+	}
+};
\ No newline at end of file
diff --git a/test/apps/custom-extension/src/client.js b/test/apps/custom-extension/src/client.js
new file mode 100644
index 000000000..6cce7e658
--- /dev/null
+++ b/test/apps/custom-extension/src/client.js
@@ -0,0 +1,9 @@
+import * as sapper from '@sapper/app';
+
+window.start = () => sapper.start({
+	target: document.querySelector('#sapper')
+});
+
+window.prefetchRoutes = () => sapper.prefetchRoutes();
+window.prefetch = href => sapper.prefetch(href);
+window.goto = href => sapper.goto(href);
\ No newline at end of file
diff --git a/test/apps/custom-extension/src/routes/[slug].mdx b/test/apps/custom-extension/src/routes/[slug].mdx
new file mode 100644
index 000000000..e31f6d9fd
--- /dev/null
+++ b/test/apps/custom-extension/src/routes/[slug].mdx
@@ -0,0 +1,5 @@
+<script>
+	import { page } from '@sapper/app';
+</script>
+
+<h1>{$page.params.slug.toUpperCase()}</h1>
\ No newline at end of file
diff --git a/test/apps/custom-extension/src/routes/_error.svelte b/test/apps/custom-extension/src/routes/_error.svelte
new file mode 100644
index 000000000..4cd55d28d
--- /dev/null
+++ b/test/apps/custom-extension/src/routes/_error.svelte
@@ -0,0 +1,3 @@
+<h1>{status}</h1>
+
+<p>{error.message}</p>
\ No newline at end of file
diff --git a/test/apps/custom-extension/src/routes/a.svelte b/test/apps/custom-extension/src/routes/a.svelte
new file mode 100644
index 000000000..0e6757b2b
--- /dev/null
+++ b/test/apps/custom-extension/src/routes/a.svelte
@@ -0,0 +1 @@
+<h1>a</h1>
\ No newline at end of file
diff --git a/test/apps/custom-extension/src/routes/const.whokilledthemuffinman b/test/apps/custom-extension/src/routes/const.whokilledthemuffinman
new file mode 100644
index 000000000..1dc9ef880
--- /dev/null
+++ b/test/apps/custom-extension/src/routes/const.whokilledthemuffinman
@@ -0,0 +1 @@
+<h1>Tremendous!</h1>
\ No newline at end of file
diff --git a/test/apps/custom-extension/src/routes/index.jesuslivesineveryone b/test/apps/custom-extension/src/routes/index.jesuslivesineveryone
new file mode 100644
index 000000000..b61b24443
--- /dev/null
+++ b/test/apps/custom-extension/src/routes/index.jesuslivesineveryone
@@ -0,0 +1,8 @@
+<h1>Great success!</h1>
+
+<a href="a">a</a>
+<a href="ambiguous/ok.json">ok</a>
+<a href="echo-query?message">ok</a>
+<a href="echo-query?p=one&p=two">ok</a>
+
+<div class='hydrate-test'></div>
\ No newline at end of file
diff --git a/test/apps/custom-extension/src/routes/unsafe-replacement.svelte b/test/apps/custom-extension/src/routes/unsafe-replacement.svelte
new file mode 100644
index 000000000..7b0b2dd2f
--- /dev/null
+++ b/test/apps/custom-extension/src/routes/unsafe-replacement.svelte
@@ -0,0 +1 @@
+<h1>Bazooom!</h1>
\ No newline at end of file
diff --git a/test/apps/custom-extension/src/server.js b/test/apps/custom-extension/src/server.js
new file mode 100644
index 000000000..7f090b869
--- /dev/null
+++ b/test/apps/custom-extension/src/server.js
@@ -0,0 +1,8 @@
+import polka from 'polka';
+import * as sapper from '@sapper/server';
+
+const { PORT } = process.env;
+
+polka()
+	.use(sapper.middleware())
+	.listen(PORT);
diff --git a/test/apps/custom-extension/src/service-worker.js b/test/apps/custom-extension/src/service-worker.js
new file mode 100644
index 000000000..c80a8f4a4
--- /dev/null
+++ b/test/apps/custom-extension/src/service-worker.js
@@ -0,0 +1,81 @@
+import * as sapper from '@sapper/service-worker';
+
+const ASSETS = `cache${sapper.timestamp}`;
+
+// `app.shell` is an array of all the files generated by webpack,
+// `app.files` is an array of everything in the `static` directory
+const to_cache = sapper.shell.concat(sapper.files);
+const cached = new Set(to_cache);
+
+self.addEventListener('install', event => {
+	event.waitUntil(
+		caches
+			.open(ASSETS)
+			.then(cache => cache.addAll(to_cache))
+			.then(() => {
+				self.skipWaiting();
+			})
+	);
+});
+
+self.addEventListener('activate', event => {
+	event.waitUntil(
+		caches.keys().then(async keys => {
+			// delete old caches
+			for (const key of keys) {
+				if (key !== ASSETS) await caches.delete(key);
+			}
+
+			self.clients.claim();
+		})
+	);
+});
+
+self.addEventListener('fetch', event => {
+	if (event.request.method !== 'GET') return;
+
+	const url = new URL(event.request.url);
+
+	// don't try to handle e.g. data: URIs
+	if (!url.protocol.startsWith('http')) return;
+
+	// ignore dev server requests
+	if (url.hostname === self.location.hostname && url.port !== self.location.port) return;
+
+	// always serve assets and webpack-generated files from cache
+	if (url.host === self.location.host && cached.has(url.pathname)) {
+		event.respondWith(caches.match(event.request));
+		return;
+	}
+
+	// for pages, you might want to serve a shell `index.html` file,
+	// which Sapper has generated for you. It's not right for every
+	// app, but if it's right for yours then uncomment this section
+	/*
+		event.respondWith(caches.match('/index.html'));
+		return;
+	}
+	*/
+
+	if (event.request.cache === 'only-if-cached') return;
+
+	// for everything else, try the network first, falling back to
+	// cache if the user is offline. (If the pages never change, you
+	// might prefer a cache-first approach to a network-first one.)
+	event.respondWith(
+		caches
+			.open(`offline${sapper.timestamp}`)
+			.then(async cache => {
+				try {
+					const response = await fetch(event.request);
+					cache.put(event.request, response.clone());
+					return response;
+				} catch(err) {
+					const response = await cache.match(event.request);
+					if (response) return response;
+
+					throw err;
+				}
+			})
+	);
+});
diff --git a/test/apps/custom-extension/src/template.html b/test/apps/custom-extension/src/template.html
new file mode 100644
index 000000000..0eb1f3ba4
--- /dev/null
+++ b/test/apps/custom-extension/src/template.html
@@ -0,0 +1,14 @@
+<!doctype html>
+<html lang="en">
+<head>
+	<meta charset='utf-8'>
+
+	%sapper.base%
+	%sapper.styles%
+	%sapper.head%
+</head>
+<body>
+	<div id='sapper'>%sapper.html%</div>
+	%sapper.scripts%
+</body>
+</html>
diff --git a/test/apps/custom-extension/test.ts b/test/apps/custom-extension/test.ts
new file mode 100644
index 000000000..501b78eb5
--- /dev/null
+++ b/test/apps/custom-extension/test.ts
@@ -0,0 +1,75 @@
+import * as assert from 'assert';
+import * as puppeteer from 'puppeteer';
+import * as http from 'http';
+import { build } from '../../../api';
+import { AppRunner } from '../AppRunner';
+import { wait } from '../../utils';
+
+declare let deleted: { id: number };
+declare let el: any;
+
+describe('custom extensions', function() {
+	this.timeout(10000);
+
+	let runner: AppRunner;
+	let page: puppeteer.Page;
+	let base: string;
+
+	// helpers
+	let start: () => Promise<void>;
+	let prefetchRoutes: () => Promise<void>;
+	let prefetch: (href: string) => Promise<void>;
+	let goto: (href: string) => Promise<void>;
+	let title: () => Promise<string>;
+
+	// hooks
+	before(async () => {
+		await build({ cwd: __dirname, bundler: 'rollup',ext: '.jesuslivesineveryone .whokilledthemuffinman .mdx .svelte' });
+
+		runner = new AppRunner(__dirname, '__sapper__/build/server/server.js');
+		({ base, page, start, prefetchRoutes, prefetch, goto, title } = await runner.start());
+	});
+
+	after(() => runner.end());
+
+	it('works with arbitrary extensions', async () => {
+		await page.goto(base);
+
+		assert.equal(
+			await title(),
+			'Great success!'
+		);
+	});
+
+	it('works with other arbitrary extensions', async () => {
+		await page.goto(`${base}/const`);
+
+		assert.equal(
+			await title(),
+			'Tremendous!'
+		);
+
+		await page.goto(`${base}/a`);
+
+		assert.equal(
+			await title(),
+			'a'
+		);
+
+		await page.goto(`${base}/test-slug`);
+
+		assert.equal(
+			await title(),
+			'TEST-SLUG'
+		);
+
+		await page.goto(`${base}/unsafe-replacement`);
+
+		assert.equal(
+			await title(),
+			'Bazooom!'
+		);
+	});
+
+	
+});
\ No newline at end of file
diff --git a/test/unit/create_manifest_data/index.ts b/test/unit/create_manifest_data/index.ts
index d21a537fa..cc5ec0ecf 100644
--- a/test/unit/create_manifest_data/index.ts
+++ b/test/unit/create_manifest_data/index.ts
@@ -165,4 +165,53 @@ describe('manifest_data', () => {
 			pattern: /^\/foo$/
 		}]);
 	});
+
+	it('works with custom extensions' , () => {
+		const { components, pages, server_routes } = create_manifest_data(path.join(__dirname, 'samples/custom-extension'), '.jazz .beebop .funk .html');
+
+		const index = { name: 'index', file: 'index.funk', has_preload: false };
+		const about = { name: 'about', file: 'about.jazz', has_preload: false };
+		const blog = { name: 'blog', file: 'blog/index.html', has_preload: false };
+		const blog_$slug = { name: 'blog_$slug', file: 'blog/[slug].beebop', has_preload: false };
+
+		assert.deepEqual(components, [
+			index,
+			about,
+			blog,
+			blog_$slug
+		]);
+
+		assert.deepEqual(pages, [
+			{
+				pattern: /^\/?$/,
+				parts: [
+					{ component: index, params: [] }
+				]
+			},
+
+			{
+				pattern: /^\/about\/?$/,
+				parts: [
+					{ component: about, params: [] }
+				]
+			},
+
+			{
+				pattern: /^\/blog\/?$/,
+				parts: [
+					null,
+					{ component: blog, params: [] }
+				]
+			},
+
+			{
+				pattern: /^\/blog\/([^\/]+?)\/?$/,
+				parts: [
+					null,
+					{ component: blog_$slug, params: ['slug'] }
+				]
+			}
+		]);
+
+	});
 });
\ No newline at end of file
diff --git a/test/unit/create_manifest_data/samples/custom-extension/about.jazz b/test/unit/create_manifest_data/samples/custom-extension/about.jazz
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/create_manifest_data/samples/custom-extension/blog/[slug].beebop b/test/unit/create_manifest_data/samples/custom-extension/blog/[slug].beebop
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/create_manifest_data/samples/custom-extension/blog/[slug].json.js b/test/unit/create_manifest_data/samples/custom-extension/blog/[slug].json.js
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/create_manifest_data/samples/custom-extension/blog/_default.html b/test/unit/create_manifest_data/samples/custom-extension/blog/_default.html
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/create_manifest_data/samples/custom-extension/blog/index.html b/test/unit/create_manifest_data/samples/custom-extension/blog/index.html
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/create_manifest_data/samples/custom-extension/blog/index.json.js b/test/unit/create_manifest_data/samples/custom-extension/blog/index.json.js
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/unit/create_manifest_data/samples/custom-extension/index.funk b/test/unit/create_manifest_data/samples/custom-extension/index.funk
new file mode 100644
index 000000000..e69de29bb