Two clocks are running, not one
Homeowners often talk about "ADU timeline" as if it's one number, but there are really two separate clocks: the permitting clock (getting your plans approved, generally up to 60 days for a complete application under state law — see our California ADU laws guide) and the construction clock (the actual building process once you have a permit in hand). This guide focuses on the construction clock — what happens after your permit is issued, and how long each phase typically takes.
Typical total construction time by ADU type
| ADU Type | Typical Construction Time |
|---|---|
| Junior ADU (JADU) | 2–5 months |
| Prefab / Modular | 3–6 months |
| Garage Conversion | 4–8 months |
| Attached ADU | 6–11 months |
| Detached ADU | 8–15 months |
These ranges reflect construction time only, starting from permit issuance — see our ADU cost guide for how these types compare on price. Faster isn't automatically better or worse; a JADU is quick because it reuses existing space, while a detached ADU takes longer because it's new construction from the ground up.
Phase-by-phase: what actually happens during construction
What actually causes delays
- Failed inspections. If an inspector flags an issue at any stage, that work has to be corrected and re-inspected before the next phase can start — this is one of the most common sources of schedule slippage.
- Material availability. Custom windows, specific fixtures, or certain finish materials can have longer lead times than the rest of the project, holding up finish work even when everything else is on track.
- Weather. Foundation and framing work is the most weather-sensitive; a wet stretch during those phases can push the whole schedule back.
- Scope changes mid-project. Deciding to change a layout or finish after construction has started almost always adds time, on top of the added cost.
- Utility company scheduling. If your ADU needs a new electrical service upgrade or utility connection, coordinating with the utility company can add weeks that aren't within your contractor's control.
How to set a realistic timeline expectation
Ask your contractor for a phase-by-phase schedule, not just a single completion date — it's much easier to spot a slipping timeline early if you know when each phase is supposed to start and finish. Build in some buffer for at least one of the delay factors above; a contractor who promises a perfectly smooth schedule with zero contingency is setting an expectation that's hard to actually hit.
Frequently asked questions
No — the ranges in this guide start after your permit is issued. Permit review is a separate clock, generally up to 60 days for a complete application under state law. See our California ADU laws guide for how permitting timelines work.
Some phases have limited flexibility — inspections and material lead times set a floor on how fast things can move. Choosing a simpler design, in-stock materials, and a pre-approved plan where your city offers one are the most realistic ways to shorten a timeline before construction begins, rather than trying to compress it once it's underway.
A detached ADU is new construction from the ground up — it needs a new foundation and full framing that a garage conversion skips entirely, since the garage's structure already exists. See our detached vs. attached ADU comparison for more on this trade-off.