How Long Does It Take to Build an ADU?

Most ADUs take 8 to 14 months from first sketch to move-in. The build itself is rarely the bottleneck — design and permitting usually take as long as construction.

Typical timeline by stage

• Feasibility: 1–2 weeks
• Design and construction documents: 1–3 months
• Permitting and plan check: 1–3 months (faster where state law mandates a 60-day ministerial review)
• Construction: 4–8 months for site-built; 2–4 months for prefab/modular

The fastest paths

A garage conversion skips foundation and shell work, often finishing construction in 2–4 months. A prefab unit overlaps factory build with on-site foundation work, compressing the back half of the schedule. The slowest path is a fully custom detached build in a city with a slow permit office.

How to avoid delays

Submit a complete application the first time, use pre-approved or prefab plans where possible, and confirm feasibility before you pay for design. The most common delay is plan-check corrections caused by missing documents.