Timeline
How Long Does It Take to Build an ADU in California?
Permits, factory build, foundation, install, finishing, a realistic week-by-week timeline for a modular ADU on a typical Bay Area lot in 2026.
A realistic week-by-week build timeline for a modular ADU on a typical Bay Area lot. Site-built projects roughly double these numbers.
Weeks 1 – 4: Discovery and feasibility
Site visit, survey, zoning check, sewer/electrical capacity review, budget alignment. Outputs a feasibility report and a fixed design fee.
Weeks 5 – 12: Design
Architectural design, interior selections, structural and Title 24 engineering. Ends with a permit-ready drawing set.
Weeks 8 – 20: Permitting
Submittal, plan check, one or two rounds of corrections, permit issuance. Varies by city. Mountain View often runs 8 – 12 weeks; Atherton can run 16 – 28 weeks.
Weeks 13 – 20 (parallel): Factory build
Modules are built in the factory while permitting is finishing. This is the time-saving move modular gives you over site-built.
Weeks 20 – 24: Foundation and site work
Permit issued → foundation poured, utility trenches, site prep. About 2 – 4 weeks.
Week 25: Install day
Modules craned onto the foundation in a single day. The most photogenic day of the project.
Weeks 25 – 32: Button-up and finish
Roof seam, exterior trim, utility hookups, interior finish, landscape repair. 6 – 8 weeks.
Week 33: Final inspection and certificate of occupancy
Build phase: 2 – 4 months from permit to move-in for a modular ADU. Overall from signed contract, most projects land in 4 – 8 months. Site-built equivalent: 16 – 20 months.
Let's build it.
A free 30-minute call with our team. You'll leave with a feasibility read on your lot and a real sense of what the project costs and how long it takes.
