Who issues ADU permits in unincorporated areas of California?

If your property is outside any city's limits, the county is your permitting agency: county planning reviews zoning and the county building department issues the building permit. State ADU law applies to counties the same way it applies to cities, including the 15-business-day completeness review and the 60-day decision deadline.

Counties are bound by the same state ADU law

Government Code section 66313 defines the local agency that must follow state ADU law as a city, county, or city and county, whether general law or chartered. In unincorporated areas, the county adopts the ADU ordinance and county departments handle review. State law also defines permitting agency broadly to include planning and building departments, utilities and special districts involved in reviewing your ADU, so a water or sewer district can be part of the process even when the county issues the permit.

Example: unincorporated Los Angeles County

• Regional Planning is the planning department for unincorporated areas and reviews the ADU against zoning standards.
• Public Works Building and Safety handles building code plan check at a district office or online through EPIC-LA. It screens the package for completeness, issues an agency referral sheet listing the other approvals you need, and issues the permit once those approvals, including Regional Planning's, are in.
• Your mailing address can mislead you. LA County notes that some unincorporated addresses show a city name and offers a jurisdiction lookup tool to confirm who has authority. Altadena, for example, is unincorporated, so Eaton fire rebuilds there go through the county.
• Contract cities. Some incorporated cities use LA County Building and Safety for building plan check. In that case the county may check your building plans while the city's own zoning rules still apply.

Issues that come up more often in county permits

Unincorporated parcels are more likely to rely on a septic system instead of a public sewer. State ADU law allows the agency to require approval from the local health officer where a private sewage disposal system is used, and, for certain ADUs connected to an onsite wastewater treatment system, a percolation test completed within the last five years, or ten years if the test has been recertified. Other county departments, such as fire and environmental health, may also need to sign off.

What this means for your MEP plans and permit set

Your plans must meet the same statewide electrical, plumbing, mechanical and energy codes in a county as in a city, plus any local amendments, but the submittal format, sheet requirements and referral agencies follow the county's checklist. On septic properties, the plumbing plans need to show the building drain routed to the onsite system and match any health department approval. Get your county's ADU checklist before drawings are finalized. See permits required for a California ADU, how ADU sewer connections work, and the California ADU permit guide. Confirm your jurisdiction and its requirements with your county planning and building departments.

Sources: leginfo.legislature.ca.gov, leginfo.legislature.ca.gov, leginfo.legislature.ca.gov, planning.lacounty.gov, dpw.lacounty.gov.