If anything wants to survive when exposed to an ever-changing environment, it needs to adapt to the change, by changing.
The same goes to websites. When they are online on the web, they are exposed to trends and demands that keep on changing.
For that reason, websites that want to thrive on the web, needs to continuously appeal its users and visitors. And one of the ways, is by having a fresh look whenever its design looks old or outdated.
But the thing about web design is that, it is more than just appealing human visitors. A web design should also appeal search engines.
As for the latter, from Search Engine Journal, someone asked Google's Webmaster Trends Analyst John Mueller on a Google Office Hours hangout, asking whether a change in only a web design's layout can affect rankings.
Mueller answered yes. Web redesign could affect rankings:
"This is something that some people work on actively as well with regards to on-page SEO."
"So things like figuring out how to use titles properly on a page, how to do internal linking properly, how to provide more context for the article itself."
"All of this can definitely affect SEO."
This should be a surprise to some, since the web redesign doesn't change the content at all, nor change its URL structure. The only thing that changed, is the layout.
So generally, the answer would be no. A web redesign shouldn't affect rankings, especially when the design isn't complex.
But Mueller's explanation went deeper than just a 'simple' web redesign.
While many professional and business websites use custom design in order to make it unique and one-of-a-kind, most websites on the web use themes that are either available for free or for purchase.
Most of those themes, have layouts that out-of-the-box, can quickly be utilized to create compelling web designs. But still, no website that has been around on the web for quite some time, should simply apply everything a theme offers.
Those websites may already have a brand name and recognition. There should be some tweaks here and there to make the theme looks as it should. Even a theme's layout that is labeled as “SEO friendly” should have its CSS and various parts of its HTML tweaked.
Web designers can tweak those things up pretty quickly But there are things like, for example, structures of the themes that use different elements, naming, or hierarchy. And these differences from one web design to another, can affect how Google sees that website.
So here, Google is like saying that redesigning a website's web design should be done correctly, and also with careful consideration. Google needs time to re-learn what it knows about a site, when its web design is changed. And this can fluctuate its rankings.
Web designers may need to work with web developers in order to make the design behave like it should with the site's features and functions.
"It can definitely affect SEO. And it can be a good thing, it can be a bad thing."
"So it’s not that you need to avoid making these changes but rather when you make these changes make sure to double check that you’re kind of doing everything really well."