To be done correctly. Just wanted to get that out of the way because you didn't say what you were planning. My uncle does this here in Ohio and it's a pretty smooth operation. Has a complete hydraulic system that lifts the house a few inches, then they dig out, build a new foundation, and sit it back down. Typically, it's much more expensive than what it adds to the homes value. And as Steve mentioned, depending on your area a detriment. But if like me your dream home includes a full basement, it can be worth it to you. I have a very old Colonial which only has a partial basement. Besides the fact that the stone foundation could stand to be replaced, I want a full finished basement regardless of cost/value. I'm looking at about $20,000 just to get started, but it'll add 1200 sq. ft without taking away from the exterior appearance. If you're serious and not concerned about recouping your money, do some research locally on who is qualified to do the work. Make sure you check out references and look at some of their work. Not properly done, it can be a huge disaster and picking the cheapest yahoo could be scary.