Cost to Build a House With an ADU in 2026

Building a main home plus an ADU typically runs $420,000–$700,000+ combined in 2026, depending on size, location and ADU type. The main home (about 2,000 sq ft at ~$170/sq ft) is roughly $340,000, and the ADU adds anywhere from $40,000 for a garage conversion to $250,000 for a large detached unit. Building both at once is cheaper than adding an ADU later.

Almost always cheaper to build both at the same time. You share site work, utilities, permitting, the general contractor and design — avoiding a second mobilization and duplicate soft costs. Adding an ADU years later means re-excavating, re-permitting and paying a crew to return, which can add tens of thousands of dollars.

Roughly: an attached ADU runs $60,000–$150,000, a detached ADU $100,000–$250,000, a garage conversion $40,000–$120,000, and a prefab/modular ADU $80,000–$200,000. Detached units cost the most but are the most flexible and command the highest rents. Costs vary widely by state — see our ADU cost guides.

Yes. A wave of 2025–2026 laws now allows ADUs by-right in many states, caps impact fees, and — in states like California — eliminates owner-occupancy requirements so you can rent both units. That makes a house-plus-ADU build easier to permit and better as an income play. Check our ADU laws by state guide for your location.

Often, yes. Lenders may count projected ADU rental income toward qualifying, and building the ADU with the main home lets you finance both under one construction-to-permanent loan. This is a popular strategy for offsetting your mortgage — model the payback with the ROI calculator before committing.