Separate mobile site vs responsive design: which is better?

Key takeaways

Separate mobile site vs responsive design is a settled question for almost every UK business: choose responsive. A separate mobile site, usually at an m. address, doubles the upkeep and creates SEO risks that a single responsive site avoids. If you still run one, plan a move.

• A separate mobile site means two sets of pages, two sets of bugs and two things to keep in step.
• A responsive site has one URL per page, so links, shares and rankings are never split.
• Content left off a separate mobile site may not be indexed, because Google indexes mobile.
• Google recommends responsive design as the easiest pattern to implement and maintain.

What is a separate mobile site?

A separate mobile site is a second version of a website, built for phones and served from its own address, typically m.example.co.uk. When a phone visits the main site, the server detects the device and redirects it to the mobile address. The two versions have different HTML and often different content.

This was a common approach around 2010 to 2014, before responsive design matured. A related method, dynamic serving, keeps one address but sends different HTML depending on the device. Both rely on correctly guessing what device is asking, and both leave you maintaining more than one version of every page.

• Why mobile-friendly web design is essential for UK businesses
• What is responsive web design? A plain-English guide

How do the three approaches compare?

Google's documentation lists three configurations for mobile: responsive design, dynamic serving and separate URLs. They differ in how many addresses and how many sets of HTML you maintain. Responsive design has one of each, which is why it is the lowest-risk option for a small team.

• Responsive: one URL, one HTML. Layout adapts with CSS. Lowest upkeep.
• Dynamic serving: one URL, different HTML by device. Needs reliable device detection.
• Separate URLs: two URLs, two HTML sets. Needs redirects and link annotations on every page.
• Content risk: none with responsive; real with the other two if versions drift apart.
• Analytics: one set of pages with responsive; split or duplicated with separate URLs.

Why is a separate mobile site an SEO risk?

A separate mobile site is an SEO risk because Google indexes the mobile version. Whatever is on the m-dot pages is what gets indexed and ranked. If those pages were trimmed to load quickly on 3G phones a decade ago, the trimmed version is now your website in Google's eyes, however complete the desktop site is.

There are mechanical risks as well. Each desktop page must point to its mobile twin and each mobile page must point back, redirects must send every device to the right place, and structured data and titles must match on both. Google's 2023 announcement that mobile-first indexing was complete named sites whose mobile pages all redirect to the homepage as one of the errors it cannot resolve.

• Mobile-first indexing: what it means for your business
• Does Google index content hidden on mobile?

What does each approach cost to run?

A separate mobile site costs more to run because every change is made twice. A new service page, a price change, a legal notice or a cookie banner update has to be applied and tested on both versions. Website care plans in the UK typically run from £50 to £500 a month, and a second site pushes the work towards the top of that range or beyond.

Moving to a single responsive site is a one-off project. Indicative UK ranges are £3,000 to £12,000 for a redesign and £5,000 to £20,000 or more for a rebuild, depending on size. The move pays back through lower maintenance and by removing the risk of the two versions disagreeing.

• Website maintenance costs in the UK
• Website redesign versus rebuild

When does a separate mobile experience still make sense?

A separate mobile experience still makes sense in a few large-scale cases: very high-traffic publishers or retailers with dedicated engineering teams, where the mobile product is different enough to justify its own codebase. Even most of those have moved to responsive or to an app alongside a responsive site.

For a small or mid-sized UK business it does not make sense. If the reason given is speed, a well-built responsive site is fast. If the reason is a simpler mobile layout, responsive design does that with one set of content. If the reason is that the old site cannot be changed, that is an argument for replacing the old site.

• Mobile website vs app: does your business need an app?

How do you move from an m-dot site to responsive?

Treat the move as a site migration. Build the responsive site on the main address, make sure every page that existed on the mobile address has an equivalent, then permanently redirect each m-dot URL to its matching page and remove the device-based redirects. Keep the redirects in place indefinitely.

• List every URL on both versions and map each to a page on the new site.
• Carry over any content that exists only on the mobile version.
• Add 301 redirects from every m-dot URL to its responsive equivalent.
• Remove the device-detection redirects and the alternate link tags.
• Update internal links, the sitemap and Google Business Profile links.
• Watch Search Console for crawl errors for at least a month.

• Website migration SEO checklist

What does this look like in practice?

A pattern we see in established local firms: a desktop site last redesigned years ago, plus a mobile version added later by a different supplier. The mobile version has five pages. The desktop version has forty. Phone visitors are redirected to the five-page site, and service pages that exist only on desktop have quietly dropped out of search results.

The owner's view is that the website has 'stopped working'. The cause is that Google now reads the five-page version. Merging the two into one responsive site, with redirects from the old mobile addresses, restores the missing pages to the version Google indexes. Global Bridge Labs (GBL) plans these moves as migrations, with a URL map agreed before any design work starts.

• See client work
• Website Design & Development for UK businesses
• Make your website work properly on mobile

Next step

If your site still redirects phones to a different address, check how many pages each version has. A large difference is the clearest sign that a move to one responsive site is overdue.