What's the fastest an ADU can realistically be built?

Prefab or modular ADU projects in California have reported total timelines of about 3 to 6 months, since the unit is manufactured off-site while permitting and site prep happen in parallel. A fully custom, site-built ADU rarely moves faster than about 6 months even in ideal conditions.

Is the 60-day ADU permit review rule the same in every state?

No. The 60-day ministerial review deadline described here is California state law under Government Code section 66317. Other states and cities have their own ADU ordinances, some with streamlined review and some without, so check your local planning department's rules directly.

Does converting a garage into an ADU take less time than building a new one?

Generally yes, because you're working with existing foundation and structure rather than new construction and new utility connections. It still has to go through the same permitting process, so the design and permitting phases can take a similar amount of time even if construction itself is shorter.

What's the single biggest thing that delays an ADU project?

Plan check corrections during permitting. Even one or two rounds of revisions can add weeks to months, because the review clock effectively restarts on resubmission. A design checked against your specific city's ADU ordinance before first submittal is the best way to avoid this.

How Long Does It Take to Build an ADU? A Realistic Timeline

Building an accessory dwelling unit sounds like a construction question, but the biggest delays rarely happen on the job site. They happen in a planning department's inbox. If you're mapping out a timeline, it helps to know which phases you can control and which ones you're mostly just waiting on.

What's the realistic timeline, start to finish?

Across published builder and design-firm estimates, most ADU projects run 6 to 18 months from the first design conversation to final move-in, with 8 to 14 months being typical for a site-built unit (BuildX; Vision ADU). That range breaks down roughly like this:

Phase	Typical duration	What's happening
Design	~2 months	Site survey, floor plan, elevations, engineering
Permitting	2 to 6 months	Plan check, corrections, permit issuance
Construction	80 to 120 days (about 3 to 5 months)	Foundation through final inspection
Prefab/modular alternative	3 to 6 months total	Factory build overlaps with site prep

Those figures are national ranges pulled from current builder cost-and-timeline guides — your local numbers will move depending on your city, lot, and contractor's backlog.

Why does permitting take so long?

This is where most of the unpredictability lives, and it's largely about paperwork speed, not construction. In California, state law requires that a complete ADU application be reviewed ministerially — no public hearing, no discretionary review — and approved or denied within 60 days of the city deeming the application complete, under Government Code section 66317 (California HCD). If a city misses that window, the application is deemed approved by operation of law.

That sounds fast, but two things stretch it out in practice:

The clock doesn't start at submission. It starts once the city marks your application "complete," and incomplete applications get bounced back for corrections, which resets the wait.
Corrections are common. Even a well-drawn plan set often comes back with plan-check comments once or twice, and each resubmission restarts part of the review.

In practice, builders report the full permitting phase — first submittal through issued permit, corrections included — running 2 to 6 months even under a 60-day statutory review clock. If you're outside California, check your own state or city ADU ordinance; many states have adopted similar streamlined-review rules, but the specific deadlines differ.

What actually happens during construction?

Once permits are in hand, on-site work for a typical detached ADU runs 80 to 120 days — about three to four months — covering foundation, framing, rough mechanical/electrical/plumbing, insulation, drywall, finishes, and final inspections. Weather, inspector scheduling, and subcontractor availability are the usual sources of slippage here, the same as any small residential build.

Does going prefab or modular actually save time?

Often, yes. Because the unit is built in a factory while your site work and permitting happen in parallel, some California prefab ADU projects report total timelines closer to 3 to 6 months, compared with 6 to 12 months for a fully site-built unit. The tradeoff is less flexibility in custom layout and finishes, and you still have to clear the same permitting process before the factory-built unit can be set and connected.

Does size or ADU type change the timeline much?

Yes, and it's one of the easier variables to plan around up front. A studio or one-bedroom detached ADU with a simple footprint generally moves through design and construction faster than a two-bedroom unit with a more complex roofline or a second story. Attached ADUs and garage conversions that reuse an existing foundation and roof structure tend to shave time off construction, since crews aren't starting from bare dirt, though they still go through the same design and permitting steps as new construction. New utility connections — a separate water line, sewer lateral, or electrical panel upgrade — are one of the more common hidden time-adders, since they can trigger review from a separate utility department on top of the building permit itself.

Where do projects actually lose months?

The pattern that shows up across builder timelines isn't usually the framing crew running slow — it's gaps between phases: weeks waiting for a plan reviewer to pick up your file, a design that goes back for revisions because it wasn't checked against your specific city's setback and height rules before submittal, or a permit that sits ready while you're still finalizing a contract with a builder. Complexity adds time too — a detached new-construction ADU with its own utility connections typically takes longer than an attached conversion of existing space, like a garage.

What to do about it

If you're trying to keep your timeline tight:

Confirm your local ADU ordinance before you design, not after. Setback, height, and parking rules vary by city, and a plan drawn against generic assumptions is more likely to get bounced in plan check.
Ask any contractor or designer for their actual recent permit turnaround times in your city, not just the statutory maximum — real-world experience with your specific planning department is worth more than the law on paper.
Line up your contractor before permits are final, so construction can start the week you're cleared rather than weeks later.
Get more than one quote before you commit. Timelines and prices both vary a lot by builder, and comparing a few side by side is the only way to know whether a bid — or a promised schedule — is actually reasonable for your project, instead of just taking one contractor's word for it.

If you're weighing whether an ADU is worth the timeline at all, it's also worth reading up on whether an ADU really increases property value and comparing it against what a garage conversion costs as a faster, lower-scope alternative.