1 Adding a basement to an existing house can cost $30,000-$70,000 or more, depending on the size of the existing crawlspace or half-basement or if the house is on a slab foundation, and how much of the work is do-it-yourself and how much is done by contractors. A finished basement typically includes all of that plus flooring, walls, ceilings, drywall, windows, and additional waterproofing and more in order to provide the desired living space. 2  Overall, a basement adds about 3-4 weeks to the new-house construction schedule.