Four ways to support an addition — slab, crawlspace, basement, or none (cantilever), cheapest to priciest
Foundation is the biggest single structural cost swing. Slab-on-grade is the cheapest baseline; a crawlspace adds ~3–8%; a full basement adds ~15–35% (the biggest swing); and “none” — a cantilevered bump-out or building over the existing floor — costs slightly less. In cold climates footings must reach below the frost line (IRC R403.1.4).
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.
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