We don't have many basements, but I use the conditioned living area. If the basement is finished and has heat/air ducts, then it would count, if not, then I would treat it like a garage, no increase in price. BTW, I do charge more for a crawl space foundation than a slab (I hate to crawl!)
The living area figure is a pretty good indicator of what you have, since every one wants the house to be as big as possible for selling and loan purposes.

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002