Isn't that big of a deal, but very expensive. My uncle's contstruction company does this quite often. I'm seriosly considering having it done to my 1890's colonial, which currently has about 1/4 of a basement. Basically they lift the house, dig out the underneath with a bobcat, replace all the foundation walls, and sit it back down. Going to cost me about $20,000 to bring my basement to a total of 1500sq ft with 14 courses of block. Would I ever increase the value by $20,000? Nope. But I want to add to the usable space of the home without sticking "wings" on and ruining the architectural value. Depends on what your goals are, as there are certainly cheaper ways to add some space, but for me were talking $25/sq. ft by the time it's finished. Just don't plan to get your money back. If I'm satisfied with what I have, I won't need to move anyway so who cares about the resale value. BTW, if you're borrowing money to do something like this, you may have a hard time.