The short answer: For a typical California ADU, plan on roughly a year from your first design meeting to the day someone moves in. A simple garage conversion built from preapproved plans can move faster, and a complex detached unit on a tricky lot can take two years or more. Construction is only part of the clock. Design, city review, corrections, utility work, and final inspections often take as long as the building itself.
“When could someone actually live in it?”
That’s usually the second question homeowners ask about an ADU, right after “What will it cost?” Maybe a parent is moving in next fall. Maybe you’re counting on rent to help with the payments. Either way, the date matters.
Here’s the mistake we see most often: people plan around how long it takes to build, and forget everything that happens before the first shovel hits the ground. Let’s walk through each stage, what California law says about how fast your city has to move, and what you can do to keep your project on schedule.
What are the stages of building an ADU?
Every project is different, but most ADUs move through the same six stages. Some overlap. Others can’t start until the one before is finished.
| Stage | What happens | What sets the pace |
|---|---|---|
| 1. Feasibility | Confirm what your lot allows, check utilities and access, set a rough budget | How quickly you gather property records and get answers from the city |
| 2. Design and plans | A designer prepares plans; structural engineering and Title 24 energy calculations are added | Custom design vs. preapproved plans, and how fast you make decisions |
| 3. City review | The city checks that your application is complete, reviews the plans, and issues corrections | State deadlines, the city’s workload, and how fast corrections come back |
| 4. Permit and contract | Fees are paid, the permit is issued, and the construction contract and schedule are finalized | Fee payment, final pricing, and ordering long-lead items |
| 5. Construction | Site work, foundation, framing, utilities, insulation, drywall, finishes, with inspections along the way | Unit type, site access, inspection scheduling, and change requests |
| 6. Final and move-in | Utility connections, final inspection, and the city’s sign-off | Utility company schedules and any last corrections |
What does that add up to in the real world? Alameda County’s ADU program tells homeowners that most ADU projects take 12 to 18 months to complete, and some stretch to 24 months or more. A statewide survey of ADU owners by UC Berkeley researchers (people who applied for permits or finished their units in 2018 or 2019) found that about two in five finished within 10 months of starting the approval process, while about one in three took longer than 15 months.
Those numbers come from before some newer state deadlines took effect, which brings us to the part most people don’t know about.
How long does the city have to approve an ADU permit?
California sets the clock, not just your city. Under state ADU law (Government Code section 66317, as updated by SB 543 on January 1, 2026):
- Completeness check: The city has 15 business days after you submit to tell you in writing whether your application is complete. If it’s missing something, it must tell you what. Los Angeles County’s planning department notes that an application it doesn’t flag within that window is automatically deemed complete.
- Decision: Once the application is complete, the city must approve or deny it within 60 days. If it doesn’t act in time, the application is deemed approved.
- If it’s denied: The city has to give you a full written set of comments listing what’s defective or deficient, and how to fix it.
- Appeals: Cities must now offer a written appeal process for incomplete or denied applications.
Two important footnotes. First, if you ask for a delay (for example, to revise your plans), the 60-day clock pauses. Second, if you apply for an ADU together with a brand-new house on the same lot, the city can hold the ADU until it acts on the house.
And remember what the clock doesn’t cover: your design time, the time you spend answering corrections, and anything outside the building permit, such as a Coastal Development Permit in the Coastal Zone.
Do preapproved ADU plans really save time?
They can, especially at the city-review stage. A 2023 state law (AB 1332) required cities and counties to set up a preapproved ADU plan program by January 1, 2025. When you apply for a detached ADU using a plan your city has preapproved (or one identical to a plan it already approved in the current code cycle), the city must act on the complete application within 30 days.
Here’s how that looks locally:
- Long Beach offers two preapproved designs through its PAADU program: a 415-square-foot one-bedroom and an 800-square-foot two-bedroom. The city says these plans can be approved over the counter, and once approved, a building permit can be issued the same day.
- City of Los Angeles runs a Standard Plan Program through LADBS. The city says using a standard plan “greatly reduces the time required for plan check,” but also warns that a pre-approved design may not work on every property, that soil conditions may require site-specific foundation work, and that every plan still gets a final review.
The tradeoff is flexibility. A preapproved plan saves design and review time, but you build what’s on the page. If your lot is odd-shaped, sloped, or tight, or you want a specific layout, custom plans may still be the better path.
How long does ADU construction take?
It depends mostly on what you’re building. A garage conversion starts with walls, a roof, and a slab already in place, so there’s less to build, as long as that existing structure is sound. A new detached ADU starts from bare ground: excavation, foundation, framing, roofing, and new utility lines from the main house.
Whatever the type, construction usually follows this order, with city inspections at key points:
- Site prep, trenching, and underground utilities
- Foundation (or slab repair and upgrades for a conversion)
- Framing, roof, windows, and doors
- Rough plumbing, electrical, and mechanical
- Insulation, then drywall
- Cabinets, fixtures, flooring, paint, and finishes
- Final utility connections and final inspection
Each inspection has to pass before the next stage gets covered up. That’s why a good schedule is built around inspections, not just crew days. If you’re still deciding between converting and building new, our guide to choosing between a garage conversion and a detached ADU compares cost, rules, and lot fit side by side.
What usually slows an ADU down?
Most delays trace back to a short list. Use this as a checklist while you plan:
- An incomplete application (missing site plan details, energy forms, or engineering)
- Changing the design after plans are submitted
- Slow turnaround on plan check corrections
- An electrical panel or service upgrade, which runs on the utility company’s schedule
- A long sewer or utility run, or a sewer line in poor condition
- Tight site access that slows excavation and deliveries
- Soil or foundation issues that call for engineering changes
- Past unpermitted work on the property that has to be corrected
- Coastal Zone, historic district, or fire hazard area reviews
- Late decisions on windows, cabinets, fixtures, and finishes
None of these is unusual. The projects that finish close to schedule are usually the ones that spotted them early.
What can I do myself, and when do I need a pro?
A lot of the early homework is yours, and it saves real time:
- Pull your property’s permit history so you know what was built legally.
- Check your electrical panel size on the main breaker or panel label.
- Read your city’s ADU page and see whether it offers preapproved plans.
- Note any special designations, such as Coastal Zone or a historic district.
- Decide what the unit is for (family, rental, home office), because that drives size and layout.
- Make your finish choices early, and answer questions from your designer and contractor quickly.
Then bring in the professionals. A designer or architect prepares plans. A structural engineer and an energy consultant handle their parts. A licensed general contractor prices the work, builds a schedule around inspections, and manages construction to the final sign-off.
That’s the work our ADU design and construction team does. DCI is a Signal Hill general contractor in business since 2001. We guide permitting, inspections, and code-compliance coordination, and we give you a step-by-step outline and line-item costing. You can see our full range of remodeling, room addition, and new home services for homeowners, too.
FAQ
How long does it take to build an ADU in California? Plan on roughly a year from design to move-in for a typical project. Simple conversions with preapproved plans can be faster, while complex detached builds can take two years or more.
How long does a city have to approve an ADU permit in California? Once an application is complete, the city must approve or deny it within 60 days, or it is deemed approved. Since January 1, 2026, cities also have 15 business days to say whether an application is complete.
Are preapproved ADU plans faster? Usually at the review stage. For a detached ADU using a city’s preapproved plan, state law requires action on a complete application within 30 days, and some cities, like Long Beach, can approve them over the counter.
Is a garage conversion faster than building a detached ADU? Often, because the walls, roof, and slab already exist. But slab, structural, or utility problems in the old garage can add design and construction time.
Can I start building before the ADU permit is issued? No. Work that needs a permit shouldn’t start until the permit is issued. Starting early risks stop-work orders and tearing out finished work.
What causes the most ADU delays? Usually decisions and approvals, not construction: incomplete applications, design changes, slow correction responses, utility upgrades, and late finish selections.
Planning an ADU? Let’s map out your timeline
If you’re thinking about an ADU and need to know when it could realistically be ready, the best next step is a site visit and a stage-by-stage plan. We’ll look at your lot, your utilities, and what you want the unit to do, flag what’s likely to slow review, and give you a line-item picture of the work so your plans are built on real dates.
Give us a call at 562-425-1050 or email us at info@desistogc.com (dcidirect.net)