Your Brooklyn basement is the cheapest square footage you will ever add, because the walls, the slab, and the roof over it are already built and paid for. That head start is why Brooklyn basement remodeling runs from roughly $50,000 for a finished rec room to more than $300,000 for a legal rental unit with underpinning. The wide range comes from the borough itself, since a Park Slope brownstone, a Bensonhurst two-family, and a Red Hook rowhouse each put different problems between you and a finished space.