The Real Cost of Building an ADU in the USA (2026)

2026 all-in cost by ADU type (600 sq ft)

• Garage conversion: $85,000–$140,000
• Attached ADU (bump-out): $150,000–$240,000
• Detached new-build ADU: $210,000–$340,000
• Prefab / modular ADU: $180,000–$260,000 (faster, similar cost)

California is the reference market

California SB 9/10 rules made ADUs by-right. Statewide costs run 20–35% above the national average, but rental income also runs 40–70% higher. Payback in LA and Bay Area coastal cities lands 8–12 years.

Where ADUs actually pencil

Any metro where 1BR rent exceeds $1,800/month and ADUs are permitted by-right. As of 2026 that includes most of CA, WA, OR, CO Front Range, greater Boston, and NY/NJ suburbs.