Bill Hartzer: Yes it will ultimately hurt the site’s rankings.
I worked on a law firm site, mesothelioma keyword rankings. Was ranking on first page for mesothelioma lawyer.
The law firm added a bunch of pages related to criminal defense, bad drugs, and some other topics. That hurt their mesothelioma lawyer rankings, pushed it to bottom of Page 2.
We put noindex on all the pages except for the mesothelioma content pages. Rankings came back, to page 1.
We got another domain and moved the other content to that domain.
Michael Martinez: I`ve done multiple topics on many sites with no disadvantage for SEO. It really comes down to how you structure the site and how you present the content.
Wikipedia is all over the place in terms of topics. It doens`t hurt for no. 1 rankings. But it`s structured to be a diverse site - not "about" any specific thing.
Scott LaPlante: I wouldn`t do it. Keep it relative to your niche.
Buth Main: In fact, Google and other search engines evaluate a whole website. On the other hand, search engines rank pages, not sites. You should create a logical information architecture of your site to help search engines "understand" content on each page.
Darren Encabo: why not create a subdomain that focuses on content that`s not related to your niche?
Jamie Silk: I don`t see how it`ll hurt your rankings.
You don`t rank a website, you rank individual web pages.
I think google is clever enough to understand what your pages are about.
Stockbridge Truslow: Right - as Michael points out - if you structure it right and do it with purpose, it doesn`t hurt, but "expands" your realm of topics.
And as Bill points out - if you do it improperly, it can kill you. Even if the things are really in the same niche.
Keep it organized properly and have meticulous linking and structure. If you`re just playing the "match the keyword" game, you`re likely to lose. If you really understand SEO and how search engines work - you can make a site that ranks for anything and, potentially, everything.
Richard Hearne: Stockbridge Truslow in other words, YMMV.
Jody Fitzpatrick: As long as you are expanding the topics to topics that are some what relevant, for example. designing backgrounds, or creating sets, could be considered LSI keywords