The ADU construction process has nine main steps: feasibility, budget and financing, design, permit submission, plan check and approval, hiring a contractor, construction, utility hookup and final inspection, then lease-up. From the first site visit to the first rent check, most Orange County and LA County projects take about 9 to 18 months.
The building itself is usually the predictable part. The delays come from feasibility surprises, plan check corrections and utility hookups. Here’s each step, how long it typically takes and what to watch for.
The ADU construction process at a glance
| Step | Typical time | Main cost drivers |
|---|---|---|
| 1. Feasibility | 2–4 weeks | Survey, zoning and utility research |
| 2. Budget and financing | 2–6 weeks | Loan type, appraisal |
| 3. Design and plans | 1–3 months | Architect or design-build firm, engineering |
| 4. Permit submission | 1–2 weeks | Plan check fees |
| 5. Plan check and approval | 2–4 months | Corrections, school and utility fees |
| 6. Hire a contractor | 2–6 weeks | Bids, contract terms |
| 7. Construction | 4–8 months | Size, foundation, finishes |
| 8. Utilities and final inspection | 1–3 months | Meter, panel and sewer work |
| 9. Lease-up | 2–6 weeks | Pricing, listing |
| Total | 9–18 months |
These are typical ranges, not promises. Your city, your contractor and your utility provider set the real schedule.
Step 1: Feasibility
Before you spend money on design, confirm the lot can actually take the ADU you want. Check:
- Zoning and lot size. State law guarantees an 800-square-foot detached ADU on most single-family lots, with 4-foot side and rear setbacks.
- Access. Can equipment and materials get to the backyard? Corner lots, alley access and RV gates make construction cheaper.
- Utilities. Where are the sewer lateral, water line and electric panel? A panel upgrade or a long sewer run can add tens of thousands.
- Easements and hazards. Utility easements, fire zones and flood zones can limit placement.
If you’re buying, do this during escrow. In my buyer process, clients connect with an ADU architect for a feasibility report while inspections and the appraisal are underway. Here’s how to verify a property is ADU eligible before you close escrow and why you should never fully trust “ADU potential” in a listing.
Step 2: Budget and financing
Set a realistic all-in budget before design. The 2025 Cost vs. Value report for Los Angeles put a backyard ADU at about $178,500. Larger two- and three-bedroom units cost more.
Common ways to pay for it:
- Cash
- HELOC on the main home
- Construction loan or renovation loan
- Cash-out refinance after completion
I compare them in How to Finance an ADU: Cash, HELOC or Construction Loan. For the full purchase-plus-build budget, see What It Really Costs to Buy a Home and Add an ADU in Orange County.
Step 3: Design and plans
You’ll choose between a custom design, a city pre-approved plan or a prefab unit. Plans typically include architectural drawings, structural engineering, a site plan and Title 24 energy calculations.
Design decisions that change the budget most:
- Size. Staying at or under 750 square feet avoids impact fees, though SB 1117 removed the old fee cliff for many projects.
- One story vs two. See Two-Story vs Single-Story ADU.
- Bedroom count. A 2-bed, 2-bath often rents for meaningfully more. See the best layout for a 2-bed, 2-bath ADU.
- Gas vs all-electric. Read Can I Have Gas in My New ADU?
Step 4: Permit submission
Submit plans to the city’s building and planning departments. Under SB 543, effective January 1, 2026, the city has 15 business days to tell you whether the application is complete. If it misses that deadline, the application is deemed complete.
How the city classifies your project, new construction, addition or alteration, affects fees and code requirements. I explain why in Is Your ADU New Construction, an Addition or an Alteration?
Step 5: Plan check and approval
Once the application is complete, the city has 60 days to approve or deny it. If it doesn’t act, the permit is deemed approved, according to California’s ADU law. A denial has to list every deficiency and how to fix it.
In practice, most delays here come from correction rounds. Each round of comments and resubmittal can add weeks. Experienced designers who know your city’s plan checkers cut this down a lot.
Fees you’ll pay around this stage:
- Plan check and building permit fees
- School fees (units of 500 square feet or less of interior livable space are generally exempt)
- Impact fees for ADUs over 750 square feet
- Water and sewer connection fees, if any
Step 6: Hire a contractor
Get at least two or three bids based on the approved plans, not on a sketch. Compare:
- What’s included: site work, utility trenching, permits, appliances
- Payment schedule tied to completed milestones
- Timeline and how delays are handled
- License, insurance and recent ADU references
Step 7: Construction
A typical detached ADU build goes in this order:
- Site prep and demolition
- Utility trenching and foundation
- Framing and roof
- Rough plumbing, electrical and mechanical, then rough inspections
- Insulation and drywall
- Windows, doors, cabinets, tile and finishes
- Fixtures and appliances
Inspections happen at each major phase. A missed inspection or failed item adds days, sometimes weeks.
Step 8: Utilities and final inspection
This is the step that surprises most owners. Electric service upgrades and new meters depend on the utility, not your contractor, and waits can stretch for months. SB 1196 sets new hookup deadlines for SCE and SDG&E starting in 2027.
After utilities are connected, the city does a final inspection and issues a certificate of occupancy. In some cases your ADU can get one before the main house is done. See when an ADU can get a certificate of occupancy first.
Step 9: Lease-up
Price the unit off real comps, not hopes. My ADU rent guide by square footage is a good starting point. Once it’s rented, you can use the lease to refinance, and the ADU’s rent may help you qualify.
Where ADU projects get stuck
- Skipping feasibility. Finding a sewer problem after design is expensive.
- Underpriced bids. The cheapest bid often leaves out site work and utilities.
- Correction rounds. Incomplete plans lead to multiple resubmittals.
- Utility waits. Plan for them from day one.
FAQ
How long does the ADU construction process take?
Usually 9 to 18 months from feasibility to rent check in Orange County and LA County. Construction alone is typically 4 to 8 months.
How long does it take to get an ADU permit in California?
Cities must confirm completeness within 15 business days and approve or deny within 60 days of a complete application. Correction rounds often push total plan check to 2 to 4 months.
What is the first step in building an ADU?
A feasibility check: zoning, setbacks, access and utilities. If you’re buying, do it during escrow.
Do I need an architect to build an ADU?
Not always. Pre-approved city plans and prefab units can shorten design, but you’ll still need site-specific plans and engineering.
What’s the biggest cause of ADU delays?
Plan check corrections and utility hookups.
The bottom line
The ADU construction process is very manageable when the early steps are done right. Confirm feasibility before you design, budget for utilities and expect a 9- to 18-month runway from start to rent check. The 60-day approval rule and the new 15-business-day completeness deadline give owners more leverage than ever.
👉 Book a buyer ADU consult call to go through your options with Dylan
Prefer to talk now? Call Dylan Serna, ADU Realtor at 714-860-2868.
Dylan Serna | The ADU Realtor | eXp Realty | DRE #02217359
This post is general information. Timelines and fees vary by city, contractor and utility provider.
