Four ways to support an addition — slab, crawlspace, basement, or none (cantilever), cheapest to priciest

What this diagram shows

Four foundation choices for an addition drawn in section on a common grade line, from cheapest to most expensive. A slab-on-grade is a thickened-edge concrete slab poured on the ground — the cheapest baseline, common in warm climates. A crawlspace sits the floor on a short stem wall and footing, adding about 3 to 8 percent to project cost but leaving plumbing accessible. A full basement is deeply excavated tall walls on a footing with a slab floor — the biggest single cost swing at about 15 to 35 percent more. None means the addition has no new footing at all: a bump-out cantilevered off the existing floor framing, or a second story built over the existing first floor, which actually costs slightly less. A dashed frost line shows that in cold climates footings must reach below it per IRC R403.1.4, deepening the dig and raising cost regardless of type.