The California ADU permit process has firm state deadlines: your city has 15 business days to tell you whether your application is complete, then 60 days to approve or deny it. A prefab ADU adds one more layer that most homeowners don’t expect, because the building itself is approved by the state while the city approves everything around it. Here is the process, step by step.
Two approvals for a prefab ADU
| What gets approved | Who approves it | When |
|---|---|---|
| The factory-built unit: structure, plumbing, electrical, energy | The state, through HCD’s factory-built housing program, using an HCD-approved design approval agency. Each unit leaves the factory with a state insignia. | Before you buy, the builder should already hold approved plans |
| Foundation, site plan, grading, drainage, utility connections, setbacks, installation | Your city or county building and planning departments | Your ADU permit application |
Ask any prefab ADU builder for their state plan approval before you sign. In our experience, if they can’t show it, your city may treat the unit as a site-built project and review every wall, which costs time and money.
Step-by-step: from idea to permit
- Pull your lot information. Get your parcel map, zoning, lot size and any easements from your county assessor or city GIS. Note sewer or septic, electrical panel size and where the water line enters.
- Check for pre-approved plans. Since 2025, every California city must run a pre-approved ADU plan program and post approved plans online. A city pre-approved plan must be approved or denied within 30 days.
- Prepare the site plan. Show the ADU footprint, setbacks (no more than 4 ft side and rear), utility routes, and the delivery and crane path for a prefab unit.
- Prepare the foundation plan. Stamped by a licensed engineer, matched to the factory’s support points.
- Submit and pay plan check fees. Keep a copy of the date your submittal was received.
- Completeness check, up to 15 business days. The city must say whether your application is complete. If it doesn’t respond in time, the application is deemed complete.
- Decision, up to 60 days. The city must approve or deny a complete application within 60 days. If it denies, it must list every deficiency and how to fix it. If it misses the deadline, the ADU is deemed approved.
- Pay fees and pull the permit. No impact fees under 750 sq ft; proportional fees above that. School fees may apply over 500 sq ft.
- Inspections. Typically foundation, utility connections, installation and final. The factory-built unit itself was inspected at the plant.
Documents to have ready
- Site plan with setbacks, easements and the ADU footprint
- State-approved factory plans with the HCD insignia information (prefab only)
- Engineered foundation plan
- Utility plan: electrical load calculation, water, sewer or septic
- Title 24 energy compliance for a factory-built (modular) unit, usually provided by the factory. HUD-code manufactured homes follow federal standards instead.
- Fire department access and, if the main home has them, sprinkler plans
- Soils report if your city or engineer requires one
Where permits get stuck
- Electrical capacity. An older 100-amp panel often needs an upgrade or a separate meter, and utility company scheduling can take longer than the city review.
- Sewer laterals and septic. Septic systems may need a capacity review by the county health department.
- Fire access. Hillside and wildfire zones can carry extra fire department conditions.
- Coastal zone. ADU law applies in the coastal zone, but a coastal development permit may also be needed. Under AB 462 (2026), cities with a certified local coastal program must act on an ADU coastal permit within 60 days.
- Incomplete submittals. Each resubmittal starts a new 15-business-day completeness review. A complete first package is the fastest path.
Frequently asked questions
How long does an ADU permit take in California?
The city has 15 business days to decide if your application is complete and 60 days to approve or deny it. Corrections and resubmittals add time, so a complete first package matters.
Do I need an architect for a prefab ADU?
Usually not for the unit itself, since its plans are already state-approved. You will need a site plan and an engineered foundation plan, and many cities require a licensed professional to prepare them.
What if my city misses the 60-day deadline?
Under state law the application is deemed approved. Keep records of your submittal date and every city response.
Want a second set of eyes on your lot before you submit? Call (800) 806-7485. Related: California ADU laws in 2026 and Can I build an ADU on my lot?
By Matt Banes, iModular.com. California modular building specialist with 38 years in the industry. Last updated: October 7, 2026. General information, not legal advice; local requirements vary.
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