Take stock of every URL and what ranks
Start with a complete list of the old site's addresses. Crawl the site, export the URLs from the current CMS and the sitemap, and add the pages Search Console and your analytics show traffic for, since a crawl misses pages without internal links and old campaign addresses. For each URL, note clicks, impressions, the queries it ranks for and the links pointing to it from other sites. The list shows which pages carry the traffic, often a small share of them all, and which can be merged or removed.
A 301 redirect map from old to new
Every old address that has traffic, links or a counterpart on the new site gets a permanent 301 redirect to the most relevant new page. Redirect one to one wherever possible: a product to the same product, a category to the same category. Google treats sending everything to the start page as a missing page, and the value the old page had is lost. Avoid chains where one redirect leads to the next, update internal links so they point straight at the new addresses, and keep the redirects in place for at least a year, preferably for good. Pages with no counterpart and no value can return 404 or 410.
Keep the titles and content that rank
A page ranks well because of its content, its title and the way it answers a query. Moving it to a new design is fine, but rewriting the text at the same time, shortening it or splitting it into tabs changes what Google sees. Carry over the titles, meta descriptions, headings and main text of the pages that carry traffic, and change them later, one at a time, when you can see the effect. The same goes for structured data, such as product and breadcrumb markup, that the old site had.
Canonical tags, hreflang and the staging site
Every new page should have a canonical tag pointing to its own final address, in the same form as the redirects and the sitemap, with or without a trailing slash and on the right domain. Sites in several languages need hreflang tags that point both ways between the language versions, plus x-default where it fits. The staging site must be closed to search engines, preferably with a password, and the block must be lifted at launch: a noindex tag or a robots.txt left over from staging is a common reason a new site disappears from Google.
Launch day: sitemap and Search Console
At launch, test the redirects against the full list of old addresses, checking the status code and destination for each one. Submit the new XML sitemap in Search Console. If the domain changes too, verify both domains in Search Console and use the Change of Address tool. Check that robots.txt allows crawling, that the pages render without errors, and that analytics and conversion tracking are in place, so you can compare before and after.
Speed and Core Web Vitals on the new site
A new design often brings heavier images, more scripts and new fonts. Measure the old site before launch so there is something to compare with, and test the new one over a mobile network. LCP, INP and CLS are Google's measures of loading, responsiveness and visual stability, and a rebuild is a good occasion to improve them. Images in the right size and modern formats, as little JavaScript as possible and fonts served from your own server do the most. In an online shop the product and category pages matter most, and e-commerce describes moving a shop between platforms.
What to measure in the first weeks
Some movement in rankings is normal in the first weeks, while Google crawls the new addresses. Follow clicks and impressions per group of pages in Search Console, compared with the same period before, along with the page indexing report and the 404 errors in the server logs. A drop that lasts for one group of pages usually points to a missing redirect, lost content or a technical block. A website review before the rebuild gives a baseline to compare with, and the rebuild itself is described under web development.