First off, to answer the request, do design changes to your website affect Google rankings?

YES.

A website that regularly has updated content looks good in the eyes of Google. Since textual content plays such a major role in the way that Google analyzes your website it's good to pay close attention to it across all your pages of your website.

Fresh, updated content shows a couple of things to Google:

The first reason is that your site isn't dead or just a "zombie" proxy to be used to launder links or traffic to protect your good sites from getting a bad reputation.

Secondly, Google interprets regularly updated content as you actively try to improve your website for your visitors. A lot of people don't realize that SEO is merely strategies to take advantage of a rating system. Search engines are meant to deliver the most relevant information to the user.

Ever changing content is also generally a good way to not only attract new visitors, but to keep them coming back. All to often you go to a good website only to find that it is rarely updated thus making it kind of useless after the first use. We'll talk more about this in just a bit.

A good way of finding out what Google has scraped from your website in terms of textual content. Check out Google's webmaster tools.

Changing your design by means of coding is another way that your Google rankings can be affected.

A point that is rarely spoken about is the code to content ratio. If you view source a page and notice that the code on an informational page dwarfs the textual content you've got a problem, or an area for improvement.

Figuring out ways to keep your design, but cleaning up your code is an excellent way to help improve your rankings. Good ways of doing this are through CSS and good coding practices.

Additionally, changing the way you've laid out your links to external and internal pages can make a difference as well. On-page placement of links is an important factor in how they're valued by Google. Placement in areas that are termed, "Gutters", will lessen the value of your links. These are areas such as the header, navigation, footer, or on pages that are link pages or similar.

If your website is easy to get around you're more likely to get more traffic obviously. What's more is that optimizing your site design and layout well help keep people on your website longer.

Basically, the point here is that your design can indirectly affect your rankings. If you present information in a great, easy to take in format with regularly updated content, you're more likely to get people to link to your website. This is a huge factor in your search engine Rankings.

Linking and more importantly and specifically, backlinks or links that are coming into your website from another website are one of the MOST important factors to getting a higher ranking within Google. To read a more indepth explanation of the types of linking, check out my other hub page.

SEO is all about split testing really. Since the algorithms used by the Search Engines are ever changing and the fact that they are quite frankly unknown to us it is wise to not put all your eggs in one basket.

Keeping a log of your changes over time is a good way to learn what works and what doesn't for your particular website. Don't go out there and just make a huge amount of changes all at once. You'll run the risk of doing something bad without knowing how to undo it.

For example, once you've gotten a page decently ranked it would be inadvisable to change the title tags and the meta description tags.

After all is said and done, making these design changes, optimizations, and tweaks should be done in an organized fashion.

Patience is key with these. If you regularly check the time interval of your website being indexed by Google it can range from a couple weeks to 6 or 8 weeks. Updated content is a good way of telling Google that it needs to come back more often because you update your website fairly often.

Realize that these aren't going to make a huge difference. Once you make the first round of good changes and see a good return in terms of increased rank you'll quicly notice that it's hard to get much farther working on design changes alone. At that point you'll really want to focus on getting links.

Design changes are a good initial step into SEO. However, at a certain point in your SEO strategy design changes become more about conversion and the visitor experience instead of trying to increase your SEO ranking.