diff --git a/src/api/build.ts b/src/api/build.ts
index 5fa9e6a..06ed54a 100644
--- a/src/api/build.ts
+++ b/src/api/build.ts
@@ -5,9 +5,7 @@ import rimraf from 'rimraf';
 import { EventEmitter } from 'events';
 import minify_html from './utils/minify_html';
 import { create_compilers, create_main_manifests, create_routes, create_serviceworker_manifest } from '../core';
-import { Compilers, Compiler } from '../core/create_compilers';
 import * as events from './interfaces';
-import validate_bundler from '../cli/utils/validate_bundler';
 import { copy_shimport } from './utils/copy_shimport';
 
 export function build(opts: {}) {
@@ -30,6 +28,7 @@ export function build(opts: {}) {
 async function execute(emitter: EventEmitter, {
 	dest = 'build',
 	app = 'app',
+	legacy,
 	bundler,
 	webpack = 'webpack',
 	rollup = 'rollup',
@@ -57,7 +56,7 @@ async function execute(emitter: EventEmitter, {
 	// create app/manifest/client.js and app/manifest/server.js
 	create_main_manifests({ bundler, routes: route_objects });
 
-	const { client, server, serviceworker } = create_compilers(validate_bundler(bundler), { webpack, rollup });
+	const { client, server, serviceworker } = create_compilers(bundler, { webpack, rollup });
 
 	const client_result = await client.compile();
 	emitter.emit('build', <events.BuildEvent>{
@@ -66,11 +65,34 @@ async function execute(emitter: EventEmitter, {
 		result: client_result
 	});
 
-	fs.writeFileSync(path.join(dest, 'build.json'), JSON.stringify({
+	const build_info: {
+		bundler: string;
+		shimport: string;
+		assets: Record<string, string>;
+		legacy_assets?: Record<string, string>;
+	} = {
 		bundler,
 		shimport: bundler === 'rollup' && require('shimport/package.json').version,
 		assets: client_result.assets
-	}));
+	};
+
+	if (legacy) {
+		process.env.SAPPER_LEGACY_BUILD = 'true';
+		const { client } = create_compilers(bundler, { webpack, rollup });
+
+		const client_result = await client.compile();
+
+		emitter.emit('build', <events.BuildEvent>{
+			type: 'client (legacy)',
+			// TODO duration/warnings
+			result: client_result
+		});
+
+		build_info.legacy_assets = client_result.assets;
+		delete process.env.SAPPER_LEGACY_BUILD;
+	}
+
+	fs.writeFileSync(path.join(dest, 'build.json'), JSON.stringify(build_info));
 
 	const server_stats = await server.compile();
 	emitter.emit('build', <events.BuildEvent>{
diff --git a/src/cli.ts b/src/cli.ts
index 41a208e..96cd811 100755
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -31,8 +31,13 @@ prog.command('build [dest]')
 	.describe('Create a production-ready version of your app')
 	.option('-p, --port', 'Default of process.env.PORT', '3000')
 	.option('--bundler', 'Specify a bundler (rollup or webpack, blank for auto)')
+	.option('--legacy', 'Create separate legacy build')
 	.example(`build custom-dir -p 4567`)
-	.action(async (dest = 'build', opts: { port: string, bundler?: string }) => {
+	.action(async (dest = 'build', opts: {
+		port: string,
+		legacy: boolean,
+		bundler?: string
+	}) => {
 		console.log(`> Building...`);
 
 		process.env.NODE_ENV = process.env.NODE_ENV || 'production';
@@ -78,9 +83,11 @@ prog.command('export [dest]')
 	.option('--build-dir', 'Specify a custom temporary build directory', '.sapper/prod')
 	.option('--basepath', 'Specify a base path')
 	.option('--timeout', 'Milliseconds to wait for a page (--no-timeout to disable)', 5000)
+	.option('--legacy', 'Create separate legacy build')
 	.option('--bundler', 'Specify a bundler (rollup or webpack, blank for auto)')
 	.action(async (dest = 'export', opts: {
 		build: boolean,
+		legacy: boolean,
 		bundler?: string,
 		'build-dir': string,
 		basepath?: string,
diff --git a/src/cli/build.ts b/src/cli/build.ts
index 6b291b6..d13c00e 100644
--- a/src/cli/build.ts
+++ b/src/cli/build.ts
@@ -4,15 +4,20 @@ import { locations } from '../config';
 import validate_bundler from './utils/validate_bundler';
 import { repeat } from '../utils';
 
-export function build(opts: { bundler?: string }) {
+export function build(opts: { bundler?: string, legacy?: boolean }) {
 	const bundler = validate_bundler(opts.bundler);
 
+	if (opts.legacy && bundler === 'webpack') {
+		throw new Error(`Legacy builds are not supported for projects using webpack`);
+	}
+
 	return new Promise((fulfil, reject) => {
 		try {
 			const emitter = _build({
 				dest: locations.dest(),
 				app: locations.app(),
 				routes: locations.routes(),
+				legacy: opts.legacy,
 				bundler,
 				webpack: 'webpack',
 				rollup: 'rollup'
diff --git a/src/rollup.ts b/src/rollup.ts
index c23014a..399c1a7 100644
--- a/src/rollup.ts
+++ b/src/rollup.ts
@@ -9,8 +9,11 @@ export default {
 		},
 
 		output: () => {
+			let dir = `${locations.dest()}/client`;
+			if (process.env.SAPPER_LEGACY_BUILD) dir += `/legacy`;
+
 			return {
-				dir: `${locations.dest()}/client`,
+				dir,
 				entryFileNames: '[name].[hash].js',
 				chunkFileNames: '[name].[hash].js',
 				format: 'esm'