How Long Does It Take to Build an ADU?

The question I get right after "how much" is "how long," and the honest answer depends on two things more than anything else: what kind of ADU you are building, and which city is reviewing it. The type sets the construction time. The city sets the permit time, and that is the number that varies the most from one address to the next.

So here are both, side by side: the timeline by type on the ranges I plan with, and every city permit timeline we have verified against the city's own pages.

ADU Timeline by Type

Total time from the first design meeting to a finished, inspected unit, at the size each type is usually built, with the engine's cost at that size so you can see what the time buys.

Source: timelines are ADU Wizard builder planning ranges from each type's guide; costs are the ADU Wizard cost engine (standard finish, all-in) and, for prefab, our California cost guide's installed ranges. Estimates, not bids.

The pattern is simple. The less new structure you build, the faster you finish. A garage conversion reuses the slab, walls and roof, so it skips most of the site work that eats the first months of a detached ADU. Well, sort of: an old garage slab or a low ceiling can add weeks you did not plan for, and that is where I see conversions run long.

ADU Timeline by Phase

Here is how a detached build usually breaks down on the jobs I run. These are planning ranges, not guarantees, and the permit row is the one your city moves the most.

Source: ADU Wizard builder estimates; the 60-day review limit is Gov. Code 66317.

Design and permits together usually take as long as the first half of construction. This is the part nobody warns you about: the months before a shovel goes in the ground are where most schedules slip, because every correction round sends the plans back to the designer and then back into the city's queue.

Prefab: Factory Time vs Permit Time

A prefab ADU changes the construction rows, not the ones before them. The factory builds the unit while your foundation goes in, which is the real time saving.

Source: ADU Wizard planning ranges from our prefab guide; the 60-day limit is Gov. Code 66317.

ADU Permit Timelines by City

Every city and state permit guide we publish, with the timeline from that guide. Each guide cites the city's own pages, and the last column says when we last checked.

Source: each linked ADU Wizard permit guide, which cites the city's published review targets where the city publishes them and labels builder estimates where it does not.

The fastest rows all have one thing in common: a pre-approved plan or an over-the-counter lane. If time is your constraint, check whether your city has one before you pay for a custom design. Our pre-approved ADU plans guide covers how those programs work.

What Slows an ADU Down

• Correction rounds. Each one sends the set back to your designer and back into the queue. Two rounds can double the permit phase.
• Utilities. A new meter, a sewer lateral or a panel upgrade runs on the utility's schedule, not yours.
• Site work. Slopes, poor soil and tight access add weeks before framing starts.
• Extra review layers. Historic districts, hillside overlays and HOA approval sit outside the ADU clock.
• Energy compliance. In California the plan set includes Title 24 energy work; see California ADU solar requirements.
• Financing draws. A construction loan pays in stages after inspections, and a slow draw can stall a crew.

Of those, financing is the one people underestimate. Our ADU loans comparison covers how a construction loan's draw schedule works next to a HELOC.

How I Would Get It Done Faster

If I were starting today and time mattered most, I would pick the type first: a junior ADU or a garage conversion finishes months ahead of a detached unit. Then I would use a city pre-approved plan if one fits the lot, get the survey and utility checks done before design starts, and submit a complete set the first time. Honestly, the single biggest time saver I know is a designer who has permitted in your city before.

Frequently Asked Questions

How long does it take to build an ADU?

From the first design meeting to move-in, 2 to 5 months for a junior ADU, 3 to 6 months for a garage conversion, 5 to 10 months for an attached ADU and 6 to 12 months for a detached ADU, on the planning ranges I use. Construction is the longest phase; permit review is the one that varies most by city.

How fast can you build an ADU?

A junior ADU carved out of existing rooms is the fastest, at 2 to 5 months, because there is no foundation and little site work. For a new unit, the fastest path is a city pre-approved plan: San Jose can issue the permit at the plan review meeting for a complete preapproved submittal, and Long Beach issues pre-approved plans over the counter.

How long does it take to build an ADU in California?

The same 2 to 5 months to 6 to 12 months by type, with one state guarantee: a city must approve or deny a complete ADU application within 60 days (Gov. Code 66317), and since January 1, 2026 it must tell you within 15 business days whether your application is complete. Correction rounds restart part of that clock in practice.

How long does an ADU permit take?

In the cities we track, from over-the-counter or same-day for a pre-approved or small single-story plan, to as long as 26 weeks for a custom San Jose plan with several correction rounds (our estimate on the San Jose guide). The table on this page lists each city with a link to its verified permit guide.

Is a prefab ADU faster to build?

Sometimes, but less than the pitch suggests. The factory build overlaps with your foundation, which saves weeks, but design, permits and site work still come first. Plan on 6 to 12 months from contract to move-in, about the same as a site-built detached unit.

For the full cost side of the same decision, start with the detached ADU guide or our ADU permit cost guide, which lists the published fees for every city we track. The Data Hub tracks the numbers as they move.