The cost per square foot can be of some benefit if you are looking at the same or very similar homes in the same development or area and you know the condition of each home. Otherwise, it is a confusing and inaccurate statistic. We as Realtors use several different factors to determine the true market value of your home. Of course, an unfinished cellar is not even considered as a basement. Computers can't always make an accurate assessment of what is valid square feet or cost per square foot.
Trulia, Zillow and the other similar sites are using broad and general brush strokes to come up with their home values and price per square foot. Some of us offer a free Comparative Market Analysis and consultation to determine your home value and options about a sale. Taking a short cut on these websites is not the best approach to getting the important answers you need.
Robert McGuire ASR
Your Castle Real Estate
Direct - 303-669-1246
If house A has 2000 SF and is listed at $300K, it's price per SF is $150. House B has 2500 SF, and is listed at $325K and is $130 per SF. So which is the better buy? Who knows? What kind of condition are the homes in? What improvements have been made? What did the houses down the street sell for?
To get back to your question about Trulia, they just pull whatever info is in the MLS. It's not that intelligent and does not "net out" basement figures. And, you should not use Trulia for getting homes' values. They are notoriously inaccurate.
Your best source of information is your REALTOR! He/she has access to the most recent sold data in the local MLS and will be able to look at homes on an "apples to apples" comparison, which Trulia does not do.
If you're not working with a Realtor, I'm happy to help!