how much will it cost to put a basement under an existing house?
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. Having a prefabricated safe room (a welded steel box, a steel skeleton with steel planks or a prefabricated unit that is bolted together on site) installed in a basement can cost $3,000-$20,000 or more, depending on materials and size.