1- Gross Living Area is a standard that is used to describe residences. It is a published standard... although there are variations, all of them have this in common.... Gross Living Area is above grade. In practical terms, that means that if any of the floor is below the outside ground level... it's a basement. It might be well finished and could be as valuable on a $/SF basis as the upper level... but, it is basement.
2- As always, part of the appraiser's job is to locate and verify sales of properties that are the most recent, proximate, and physically similar to the subject as possible. Appraisers don't get to make the sales, we have to find them. Sometimes, there simply aren't any sales that are highly similar to the subject property. When there aren't, adjustments are made for the differences. All adjustments should be supported and ultimately are related to market reaction.