Prefab ADU Foundations: Slab, Pier, or Crawlspace for Your Lot
Your contractor handed you a line item called "foundation allowance" and told you the real number comes later. That later is usually after permits are pulled, grading is done, and you're already committed to a project that just got $15,000 more expensive. Foundation type isn't a detail -- it's the decision that shapes every cost and timeline estimate that follows. This post maps the three foundation options -- slab, pier, and crawlspace -- to specific lot conditions so you know which foundation your prefab adu actually needs before you sign anything. Which Foundation Works Best for a Prefab ADU? Slab, pier, and crawlspace each solve a different problem. The right pick depends on your soil, slope, and local code -- not on which one looks cheapest on paper. Foundation Best For Typical Cost Range Trade-offs Concrete slab Flat, stable, well-drained lots $6,000 -- $14,000 No under-floor access; poor on expansive soils Pier and beam Sloped lots, soft or uneven terrai...