The short answer
How long does it take to build an ADU? Nine to twelve months from first call to certificate of occupancy for a detached unit in Los Angeles. Here is the month-by-month breakdown and the five things that actually cause delays.
Nine to twelve months, first call to certificate of occupancy, for a detached ADU in Los Angeles. A garage conversion runs about three months shorter. Anyone promising six months for a ground-up detached unit is either not counting the permit or is about to disappoint you.
The counterintuitive part is that construction is not the long pole. Design and plan check take roughly as long as the build, and they are the phase where delays are hardest to recover.
Month by month, detached ADU
| Phase | Duration | What happens |
|---|---|---|
| Feasibility | Week 1 | Parcel study, site visit, soils review, sewer camera |
| Design | Weeks 2 to 6 | Architectural set, structural calcs, Title 24, owner approval |
| Plan check submittal | Week 7 | Filed with LADBS |
| First plan check response | Weeks 11 to 15 | Corrections come back |
| Correction cycles | Weeks 15 to 22 | One to two cycles, 2 to 4 weeks each |
| Permit issue | Weeks 22 to 24 | Clearances, fees paid, permit pulled |
| Foundation and underground | Weeks 25 to 29 | Excavation, footings, plumbing rough, slab |
| Framing and envelope | Weeks 29 to 35 | Framing, shear, roof, windows, exterior |
| Rough MEP | Weeks 35 to 39 | Electrical, plumbing, mechanical, three inspections |
| Insulation and drywall | Weeks 39 to 43 | Title 24 HERS verification, nail inspection, tape |
| Finishes | Weeks 43 to 49 | Flooring, cabinets, fixtures, paint, appliances |
| Final and CofO | Weeks 49 to 52 | Final inspection, utility connection, certificate |
Garage conversion timeline
Roughly 6 to 7 months total. Design runs 3 to 4 weeks instead of 5, plan check is lighter because the structure exists, and construction is 3 to 4 months rather than 5 to 7. The variable that can blow it up is ceiling height: if the existing structure sits below 7 feet 6 inches, raising the roof adds structural work and puts the timeline back toward a new build.
The five things that actually cause delays
1. An incomplete first submittal
This is the single biggest one. A submittal missing a Title 24 report, a structural calc page or a required stormwater sheet does not get a partial review. It gets a correction letter listing what is missing, and you have lost four to eight weeks for nothing.
2. Utility scheduling
LADWP service upgrades run on their own calendar. If the ADU needs a 200-amp upgrade, get in the queue at permit issue, not when the electrician is ready. Owners routinely lose a month here.
3. Owner-driven design changes after permit
Moving a wall after the permit is issued means a revision submittal and another review cycle. Changes are effectively free during design and expensive after plan check. This is why we hold a formal owner approval before filing.
4. Inspection scheduling
LADBS inspection availability varies through the year and can add several days per stage. Across eight inspected stages that compounds into weeks. Filing the request the same afternoon a stage completes is the only real lever.
5. Long-lead materials
Windows, custom cabinetry and some HVAC equipment can run 10 to 16 weeks. Ordering them at permit issue rather than when the framing is done is the difference between them arriving early and them stopping the job.
What can be run in parallel
- Financing approval runs alongside design, not after it
- Soils report and sewer camera run during the first week, not after design
- Prefab factory build runs concurrently with plan check, which is where prefab genuinely wins on schedule
- Long-lead material ordering starts at permit issue
- LADWP service upgrade application is filed at permit issue
The Standard Plan shortcut
Los Angeles pre-approves a set of ADU designs through its Standard Plan Program. Using one removes the architectural plan check entirely, leaving only site-specific review. That typically saves about six weeks and around $6,000. The constraint is that a pre-approved plan has to fit your lot's setback geometry, which is the first thing to check.
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Common questions
Can an ADU be built in six months in Los Angeles?
A garage conversion, sometimes, if the design is simple and plan check goes cleanly. A ground-up detached ADU, essentially never, because permitting alone typically consumes three to five months.
What is the longest part of the process?
LADBS plan check, at three to five months including correction cycles. It is longer than most people expect and longer than the framing and envelope phase of construction.
Can I speed up plan check?
Yes, in two ways. File a genuinely complete first submittal, which is the biggest single lever. And use a Los Angeles Standard Plan if one fits your lot, which removes the architectural review entirely.
Do I need to move out during construction?
No for a detached ADU. Attached ADUs and junior ADUs involve short periods of noise and scheduled utility shutoffs, but almost nobody needs to move out for either.