Prefab ADU Cost: Is a Modular Backyard Home Actually Cheaper?

Prefab and modular ADUs promise a faster, more predictable build, and they often deliver — but 'the unit price' you see advertised is rarely the all-in cost. Expect $180–$300 per square foot turnkey once site work is included.

The price you see vs the price you pay

A prefab company might quote $120,000 for a 500 sq ft unit. That's the box. On top of it you'll pay for the foundation, delivery and craning the module into place, utility trenching and hookups, permits, and any grading. Those line items commonly add 30–50% to the advertised price.

Where prefab wins

• Speed: factory and site work happen in parallel, so timelines are often half that of site-built.
• Price certainty: most of the cost is fixed before installation.
• Quality control: factory conditions reduce weather delays and rework.

Where site-built wins

If your lot is hard to access (a crane can't reach the backyard), or you want a fully custom layout, site-built can end up cheaper and simpler. Tight urban lots are the classic case where prefab's delivery advantage disappears.