A website redesign makes sense from time to time for a variety of reasons such as increasing on-site conversion ratios and making a site faster and more accessible to search engines. However, redesigning a website can damage organic search presence if SEO is not taken into consideration when changes are made to the following:
H1-H6 Tags
Page titles and headings. Valuable keywords can be placed here so, if changing the design includes changing the keywords, it could be a problem.
Images
Alt tags as well as image placement.
Internal Linking
This affects the flow of any given site’s PageRank, Google’s official metric used for ranking.
URL Structure
Similar to internal linking, how your site’s URL structure can affect it’s ranking performance (silo structured website).
John Mueller of Google went into a little more detail on this via twitter recently:
“If you change design, you generally change the content: headings, titles, images, internal linking, URL structure, accessibility, speed, etc – they’re all critical parts of a site and I’d consider them as content too.” (source)
In conclusion, with any changes being made during a website redesign, it is important to note the organic rank consequences of these changes so that it can be properly determined if the changes should be made at all.