The honest answer: 8–14 months total
Most permitted detached ADUs in California take 8–14 months from signed contract to certificate of occupancy. Garage conversions are usually faster at 5–9 months, and JADUs can sometimes close in 4–6 months total.
Phase-by-phase breakdown
Design and engineering: 6–10 weeks for a detached ADU, 3–5 weeks for a JADU. Permit plan check: 8–16 weeks in LA city; some cities are faster and California's state ADU law caps review at 60 days for compliant submissions. Construction: 16–28 weeks for a detached ADU; 12–20 weeks for a garage conversion. Final inspections and utility activation: 2–4 weeks.
What slows projects down (and what speeds them up)
The biggest delays come from incomplete permit submissions, change orders during construction, utility upgrade scheduling, and waiting on inspectors. The biggest accelerators are an experienced design-build team, complete drawings on first submission, and clear homeowner decision-making during selections.