Anonymous wrote:What kind of square footage? Not sure what types of contractors you are talking to, but this is not a small job. It's not adding-a-level-to-your-house big, but taking up the floor, moving that much dirt around is not quick and easy. If you are like most rowhouses they won't be able to get a skid steer loader down there, so that means the old floor and every bucket of dirt is coming out by hand. The old floor is probably not very thick and with no sub-surface drainage meaning they will have to remove dirt anyway to put in gravel and a vapor barrier (usually 4" gravel + new 4" floor = excavating down 8" at least). Plus removal for the french drains and sump pit. There are a ton of companies that specialize in this sort of thing, look around for contractors that specialize in underpinning; the scope of this is basically the same without the foundation re-enforcement. Alternatively a lot of concrete/hardscaping firms would be well equipped. Do you have an Angie's List account? Get a half dozen estimates from underpinning contractors from there as a start.
If they're going to go through all this effort, they might as well dig down enough to create a legal rental apartment.
People have posted on PoPville that the dig out plus re-slabbing costs about $40K. This is a very big, time intensive job.