Currently--about once every week or two--Google runs what are known as SEO Office-Hours, which are Q&A-style sessions where webmasters and entrepreneurs get to ask a Google employee (John Mueller) questions pertaining to Google search.
During the English Google SEO Office-Hours From November 5, 2021, John addressed a question from someone who had a major redesign:
“We run a small website with less than 200 URLs. A few weeks ago, we performed a major redesign to a headless CMS to improve the quality of the user experience.
“Our traffic and rankings have dropped significantly a few days later.
“How long will it take for Google to reassess the new design and return our site to its previous search position?
“Our URLs appear to be crawled post-update in Search Console. The migration used the same content URL structure metadata and navigation, with only a few technical issues--for example, the H2 headers were not shown in the HTML, which may have caused some confusion, but which were resolved two weeks later.”
Try to Maintain Structure, While Changing Cosmetics
So, from listening to John’s response (part of which I’ve edited for clarity below), it makes sense to me that you’d want to maintain as much of your site’s technical structure as possible (ie: same URLs, categories, internal links), while making cosmetic changes (graphics, visual layout, color scheme, etc).
“So, it's hard to say what all was involved, but in general, with bigger site relaunches, it is important to watch out that, as much as possible, you can reuse the old things if you really want to kind of be seen as a new iteration of the old website. So, using the same URLs--sounds like you're doing that.
“Things like internal linking...they matter...the text on the pages, the headings, the overall structure of the pages--all of that matters a little bit as well.”
Possible Technical Issue?
“...From just from looking through this question (because I don't know the website), my feeling is that there's more of a technical issue that's involved here that's causing these problems, rather than something subtle from the redesign that you launched there.”
“So, especially if you're saying you have the same URLs and the URL structure is the same, the text is mostly the same, and you're seeing a 60% drop in traffic from search, to me that would point more at some kind of a foundational technical issue that maybe popped up together with the redesign.
“[That] could be...I don't know... maybe we can't crawl your site at all anymore, or maybe we can't reach your server properly, or maybe the hosting setup that you're now using is detecting Googlebot as a rogue bot and blocking Googlebot.”
I would just like to interject here: John proposed that “maybe we can’t crawl your site at all anymore,” but actually, if you look at the original question, the webmaster said that, “Our URLs appear to be crawled post-update.”
Perhaps John was just giving a more generalized answer for others who may be in a similar situation, but in this specific situation, it seems that Googlebot was able to crawl the site.
Anyway, John continued:
“Those kinds of things are elements that I would look at here. If this were a redesign where you just changed everything (new URL structure, you set up some redirects), and it's a completely different website essentially, then I would expect this kind of a drop or a change, but if essentially everything is the same, and small things have changed, then I wouldn't expect that big of a drop and it would be more of a technical thing.”
Makes Sense...But Why the Drop In Search Traffic?
So, if you’re doing a major cosmetic change to your website, do your best to keep as much of the technical structure as similar as possible to your previous iteration...especially if you want to maintain rankings.
This means trying to keep the internal links, URLs, page names--even, to some extent, text--as similar as possible to the previous version.
Now, there may be times where you have to compromise--perhaps a previous product or service is not available, or you’re doing a rebrand, and some of your URLs need to change to reflect that.
That’s fine: things like 301 redirects can help.