Why rankings drop after a redesign
Google doesn't rank your website as a whole. It ranks individual pages, each at its own address. Over time it learns that the page at yoursite.co.uk/boiler-repairs is a decent answer for someone searching for boiler repairs in your town. If a redesign moves that page to a new address and nothing points from the old one to the new one, the search result now leads to an error. Google notices, drops the page, and has to find the new one from scratch.
Changed addresses aren't the only culprit. Redesigns often trim text to make pages look cleaner, fold several service pages into one, or go live with a setting still switched on from testing that asks search engines to stay away. Any of these can cost you visibility, and none of them are obvious from looking at the new design.
Write down what the old site does before you change anything
Start with a list of every page on the current site. On a small site you can click through and copy each address into a spreadsheet. On a bigger one, the free version of a crawler such as Screaming Frog will list up to 500 addresses for you.
Next, find out which of those pages matter. The Performance report in Google Search Console shows which pages get clicks from search, and which searches they get them for. If you've never set Search Console up, do it now, before the redesign starts, so you have a record to compare against after launch.
Finally, check you can get into everything: the domain registrar, the hosting, Search Console and your analytics. A redesign can stall for weeks over one missing login, so sort these out before any design work starts.
Keep the addresses that already work
The simplest way to protect a page's ranking is to leave its address alone. If /boiler-repairs brings in visits, the new boiler repairs page can live at /boiler-repairs too. A new design doesn't need new addresses.
Sometimes an address does have to change. Old sites often have pages ending in .php or .html, or a structure that no longer fits the business. That's fine, as long as the old address is redirected.
Redirect every old page to its closest match
A 301 redirect is a permanent forwarding instruction. Anyone who visits the old address, whether that's a person or a search engine, gets sent straight to the new one, and Google takes it as a strong sign that the new page has replaced the old.
Map each old page to the new page that covers the same thing. It's tempting to send everything to the homepage, but Google tends to treat a redirect to an unrelated page much like a missing page, and someone looking for your boiler repair prices doesn't want to start again from the front door.
We do this on our own site. When we reorganised the trade pages here, every old address got a permanent redirect to the page that replaced it rather than being left to show an error.
If you'd rather hand the whole job over, our website redesign service builds this checklist into every project, from the first audit to the checks after launch.
Carry the words that rank across
Look at the searches Search Console shows for your busiest pages. If your old gutter cleaning page gets found for 'gutter cleaning Dronfield', the new page still needs to say what you do and where, in plain words.
That doesn't mean keeping every clunky paragraph. It means rewriting with those searches in mind rather than trimming for looks. A short page can rank perfectly well. A page that no longer mentions the service or the town usually won't.
The launch-day checks
Most launch problems are small and boring, which is exactly why they get missed. On the day the new site goes live, remove any 'noindex' tag or password left over from testing, and check the robots.txt file isn't blocking the whole site. Then work through your redirect list and try each old address, starting with the ones Search Console showed getting the most clicks.
After that, test the things that bring in money. Send a real enquiry through every form and check where it lands. Tap every phone number and email link on an actual phone. Then submit the new sitemap in Search Console and make sure your analytics is recording visits, so you'll know if anything changes.
The first month after launch
Expect some movement. After any big change, rankings can wobble for a few weeks while Google re-crawls the site and works out what's moved. That's normal. A steady slide that's still going after a month or so isn't, and it's worth digging into.
Once a week, open the Page indexing report in Search Console and look for new 'Not found (404)' errors. Each one is an old address that something is still trying to reach. Add a redirect for any that matter and that problem goes away.
Leave the redirects running. Google's guidance is to keep them for at least a year, and there's little reason to ever take them down: links from other websites and old bookmarks can keep arriving long after launch.
Doing it yourself or getting help
On a five-page site, you can do all of this in an afternoon with a spreadsheet and some patience. On a bigger site, or one where you can't get into the hosting, it's worth paying someone who's done it before.
Whoever you hire, ask one question before you pay a deposit: what happens to my old page addresses? A good designer will answer straight away and in detail. If the answer is vague, keep looking.