For most San Diego homeowners, a detached ADU takes 10 to 18 months from the first design meeting to the day someone moves in. A garage conversion is usually faster — often 7 to 12 months — because there is already a structure and a foundation on the ground.
That total splits into three phases that behave very differently:
| Phase | Typical range | What controls it |
|---|---|---|
| Design and engineering | 2–5 months | ADU type, site complexity, how fast you make decisions |
| Permit review | 3–8 months | Jurisdiction, submittal quality, number of correction rounds |
| Construction | 2–7 months | ADU type, site access, material lead times, inspections |
| Total | 10–18 months | Garage conversions sit at the low end; detached and two-story at the high end |
The phases do not simply add up to the total, and that is deliberate. Real projects carry gaps between them — the pause between design approval and submittal, the wait between permit issuance and mobilization, the scheduling lag on a final inspection. Those gaps are where the difference between a 12-month project and an 18-month one usually lives.
The rest of this page explains where that time actually goes, and — more usefully — which parts of it you can influence and which you cannot.
The 60-day law, and why your permit still takes months
This is the single most confusing part of an ADU timeline, so it is worth getting straight before anything else.
California law is genuinely aggressive here. Under Government Code section 66317, a permitting agency “shall either approve or deny the application to create or serve an accessory dwelling unit within 60 days from the date the permitting agency receives a completed application.” If the agency misses that window, the application “shall be deemed approved.” If the applicant asks for a delay, the 60-day clock is paused for the length of that delay.
So why do San Diego homeowners routinely report three to eight months in permitting?
Because of two words in that sentence: completed application. The 60-day clock only starts once the jurisdiction agrees your submittal is complete and correct. In practice, plan check almost always produces at least one round of corrections across building, planning, fire and utility reviewers. Each time your plans go back for correction and are resubmitted, you are in a new review cycle — and the calendar keeps moving even though the legal clock is behaving exactly as written.
The law is not being ignored. It is being measured from a starting line most homeowners assume they have already crossed.
Phase one: design and engineering (2–5 months)
This phase covers concept development, floor plans, and — depending on the project — structural engineering, Title 24 energy compliance and utility routing.
The ADU type is the biggest driver. A garage conversion with existing walls, an existing slab and a nearby panel needs far less engineering than a ground-up detached unit, and far less again than a two-story ADU that requires structural analysis of the floor below it.
The second driver is you. Design time is the one phase where a homeowner’s own decision speed shows up directly on the schedule. Projects that stall here almost always stall on selections and layout changes, not on the designer.
There is a shortcut worth knowing about. The City of San Diego accepts three sets of preapproved ADU plans — the County of San Diego Standard ADU Plans, the City of Chula Vista Standard ADU Plans, and the City of Encinitas Permit-Ready ADU Plans. Choosing one of these compresses design work substantially, because the hard engineering is already done. You still pay the plan provider a licensing cost and you still need site-specific drawings, but you are adapting a finished design rather than originating one.
Phase two: permit review (3–8 months, or 30 days on the fast track)
Here is the fact most San Diego ADU timeline articles miss entirely.
The City of San Diego states that “applications that utilize preapproved plans will be subject to a 30-day review.” Not 60. Thirty.
That is the largest single lever available on an ADU schedule, and it is a design decision made months earlier. If your property can accept one of the three accepted preapproved plan sets, you are choosing a 30-day review track. If you want a custom ADU, you are choosing the standard track and the correction rounds that come with it. Both are legitimate choices — but they should be made knowingly, at the design table, not discovered at plan check.
Once review is finished, permit issuance itself is quick. The City’s published permit processing timeline puts building permit issuance at roughly two business days after review completes. Issuance is not where projects lose time; review is.
The second clock nobody mentions: the coastal zone
If your property sits in the coastal zone — much of La Jolla, Del Mar, Encinitas, Carlsbad and the Oceanside shoreline — there is a separate permit running alongside your building permit.
The California Coastal Commission is explicit that “ADU law does not override or supersede the Coastal Act’s requirement for a coastal development permit.” A Coastal Development Permit can still be required for an ADU, under Public Resources Code sections 30106 and 30600.
The good news is that state law gives the CDP its own discipline. Where a jurisdiction has a certified Local Coastal Program with ADU provisions, it must approve or deny a CDP application for an ADU within 60 days of a completed application, and it is not required to hold a public hearing on it. Where there is no certified local program, the Coastal Commission itself carries that 60-day obligation.
Practically, this means a coastal ADU is not automatically a nightmare — but it is a second review path with its own completeness standard, and it needs to be identified in week one, not discovered in month five. Our La Jolla ADU page and Carlsbad ADU page go deeper on how each of those jurisdictions handles it.
Phase three: construction (2–7 months)
Once permits are issued, the build itself is the most predictable phase — which is exactly why it is the phase a good contractor should be able to schedule tightly.
| ADU type | Typical active construction |
|---|---|
| Garage conversion | 2–4 months |
| Detached ADU, single story | 4–7 months |
| Two-story or structurally complex ADU | 6–9 months |
A detached ADU runs through more sequential steps — excavation, foundation, framing, rough systems, inspections, insulation, drywall, finishes, final inspections — and each one has an inspection gate in front of it. A conversion starts with a structure already standing, which removes the longest-lead early stages entirely.
Three things drag construction schedules in San Diego specifically: site access on tight or sloped lots, utility connection and upgrade timing, and long-lead material or appliance selections that were not locked before the build started.
What actually moves your timeline
Sorted honestly by how much control you have:
- ✓Choosing the ADU type that suits your lot — the biggest single factor, decided before anything else happens
- ✓Using a preapproved plan set where it fits — converts an open-ended review into a stated 30-day review
- ✓Submitting a genuinely complete package the first time — this is what starts the 60-day clock
- ✓Locking material and fixture selections before construction begins — mid-build changes cost weeks, not days
- ✓Identifying coastal, historic or utility triggers in week one — these add review paths, and finding them late is what turns a 12-month project into an 18-month one
- ✓Jurisdiction workload — real, and entirely outside anyone’s control
The first five are decisions. Only the last one is luck. Most ADU projects that run long did not get unlucky — they made a slow decision early and paid for it at plan check.
A realistic calendar, start to finish
For a detached ADU in the City of San Diego, custom design, no coastal overlay:
- 1Months 1–3: design, engineering, energy compliance, site-specific drawings
- 2Month 4: submittal to plan check
- 3Months 4–8: review, corrections, resubmittal, approval
- 4Month 8: permit issuance, mobilization
- 5Months 8–14: construction through final inspection
- 6Month 14–15: final sign-off, utilities, move-in
Swap in a preapproved plan set and a garage conversion, and the same sequence realistically compresses toward the 8–10 month end. Add a Coastal Development Permit and a second correction round, and it stretches past 18.
San Diego ADU timeline FAQs
Getting a real timeline for your property
A blended average is useful for planning and useless for committing. Once we have looked at your lot, the ADU type it can realistically support, your jurisdiction, and whether a coastal or utility path applies, we can give you a schedule with real dates on it — tracked in Buildertrend once construction starts, so you always know which stage you are in rather than guessing.
Schedule a consultation and we’ll walk through what a realistic timeline looks like for your specific property, including whether a preapproved plan set and its 30-day review are an option for you.
