ADU builders and rules in Unincorporated San Diego County

Who follows County ADU rules

The County's ADU rules apply only to property in the unincorporated areas of San Diego County: communities such as Lakeside, Spring Valley, Ramona, Fallbrook, Alpine, Valley Center, Julian, Bonsall and Jamul. If your property is inside an incorporated city, that city runs your permit instead; see our pages for the City of San Diego and Poway. A mailing address doesn't settle it, so check your zoning on the County's Property Summary Report (you need your assessor's parcel number) before you design anything.

San Diego County ADU requirements

The County's ADU standards live in Zoning Ordinance § 6156 x and are summarized in its ADU and JADU handout (PDS-611), last revised September 2, 2026. On a lot with an existing or proposed single-family home you can add a new detached ADU, a conversion ADU and a junior ADU. The County's April 2024 ADU Handbook explains each type with drawings.

Two rules matter more here than in the cities. The 25-foot height limit for a larger detached ADU is generous, so a two-story ADU or an ADU over a garage is often possible; see our two-story ADU guide. And the parking rule bites in rural areas: most of rural lots are far from a transit stop, so a new detached ADU of 800 sq ft or more there usually needs one space, though it can be tandem in your driveway.

The handout also lets you convert large permitted outbuildings. Its own example: an existing, permitted 3,000 sq ft barn could become an ADU with no size limit, as long as it meets fire and building codes for a home. A garage conversion keeps its existing setbacks, and you don't have to replace the lost parking. More in our garage conversion ADU guide.

Does the County ADU ordinance comply with state law?

Not fully. On July 7, 2026, the state housing department sent the County formal findings that its ADU ordinance breaks state ADU law in five ways. HCD said the County must measure sizes as “interior livable space,” must allow at least one larger or attached ADU alongside the state's guaranteed units, may require owner occupancy for a junior ADU only when it shares a bathroom with the house, can't bar a junior ADU next to an ADU from ministerial review, and can't require replacement parking when a garage becomes a junior ADU. The County had until August 6, 2026 to respond. HCD had also found the County's 2023 ordinance out of compliance in a September 2024 letter.

Free San Diego County pre-approved ADU plans

The County offers free dwelling unit plans that work as a detached ADU anywhere in the unincorporated area. Each comes as a 24×36 PDF, an 11×17 PDF and a CAD file. The plans are about 85% complete; you fill in the site-specific parts.

When the County released the first plans, it said they were designed and reviewed for Building Code compliance and that you would still need a site plan, a stormwater plan, energy calculations and a truss design from a truss manufacturer. The County's checklist guidance adds roofing, insulation, attic venting and, in a wildfire area, fuel modification and possibly noncombustible siding. Plans G and H on the same page (1,728 and 1,500 sq ft) are for a main house only, not an ADU.

For how the County plans compare with Chula Vista's, Encinitas's and other programs, see our guide to pre-approved ADU plans in San Diego and ADU floor plans.

Septic, slopes and fire zones

ADUs on septic

Much of the unincorporated county has no public sewer. The County's environmental health department treats an ADU as a second dwelling and says it would, in almost every case, exceed what an existing septic tank and leach field can handle. Feasibility depends on its approval of an OWTS (septic) layout application, which a septic professional prepares after percolation testing. Minimum tank sizes in its FAQ run from 1,000 gallons (one bedroom in each home) to 1,500 gallons. Get that approval before you apply for a building permit: PDS rejects site plans that don't show the well and septic locations, and the layout approval expires after a year.

Fire hazard zones and sprinklers

Contact your fire agency early. The County strongly encourages applicants to ask the fire department about fire setbacks, sprinklers and whether a fire hydrant is needed. The County's ADU Handbook says the fuel modification area is 100 feet around structures (or to the property line), with the first 50 feet cleared and planted with irrigated, fire-resistant plants. Fire sprinklers aren't required in an ADU if they weren't required for the main house. If your parcel's fire hazard severity zone is anything other than “Urban Unzoned,” the plan checklist says your site needs fuel modification.

Slopes and grading

The handbook calls flatter areas (under 10% slope) the best places to build and advises against grading slopes over 25%. Some projects need a grading permit from Land Development. Retaining walls, long utility runs and septic work are where rural ADU budgets grow; see ADU construction costs.

San Diego County ADU fees

From January 9, 2019 to January 9, 2024 the County ran a trial impact fee waiver for ADUs. It has ended. Projects started before the cutoff skipped the plan check fee, but any permit not issued by January 9, 2024 pays all fees collected at issuance. Today you pay these:

By our arithmetic from the fee schedule, County plan review and permit fees come to about $4,200 for a 600 sq ft ADU, $4,400 for 800 sq ft and $4,790 for 1,200 sq ft, before any other agency's fees. The County's Green Building program cuts plan check and permit fees by 7.5% for projects that beat state energy standards by 15% or more.

Park fees vary by community. For a larger ADU, the 2026 table lists $4,916 in Lakeside, $4,875 in Ramona, $5,017 in Fallbrook, $4,930 in Valley Center and $5,431 in Spring Valley. Under state law, impact fees on an ADU over 750 sq ft must be proportional to the size of the main house, so ask PDS how it will scale your fee. The County's online fee estimators cover building, park, traffic and drainage fees. For total project budgets, see what an ADU costs in San Diego or run the ADU cost calculator.

Getting a County ADU permit

The Building Division is at 5510 Overland Ave., Suite 110 in San Diego, open weekdays 8 a.m. to 11:45 a.m. and 12:30 to 4 p.m.; call (858) 565-5920. ADU questions go to PDS.ADUQuestions@sdcounty.ca.gov. The County's permit steps for a new dwelling run roughly like this:

• Research first. Pull your Property Summary Report, confirm you have a legal lot, and check with environmental health, zoning, fire and Land Development about septic, setbacks and grading.
• Pre-submittal review. Bring your plans to the Building Plan Pre-Submittal Review counter for zoning, building, Land Development and environmental health sign-offs, then book a submittal appointment.
• Plan check. Pay the plan check fee; review doesn't start until you do. The County says the first review backlog is about four weeks for residential projects, and about half of projects need more than two rounds.
• Clearances and fire review. Collect school, water and sewer clearance letters and get your fire district's stamp on the plans.
• Permit. Pay the remaining fees and pick up the permit. A building permit expires after two years but can be extended, per the County handbook.

After your application is complete, state law gives the County 60 days to approve or deny it. You can track records and schedule inspections in the County's Accela Citizen Access portal through Online Services Central. Before you sign a contract, read how to hire an ADU contractor and check a contractor's license.

Unpermitted ADUs and selling an ADU separately

If you have an unpermitted ADU or junior ADU built before 2020, the County says you may qualify under AB 2533 for a permit as long as it meets basic health, safety and building standards; you'll need plans showing the upgrades. Before you apply, you can get a confidential third-party code inspection from a licensed contractor.

On March 4, 2026, the Board of Supervisors voted unanimously to let owners in unincorporated communities sell an ADU separately through a condominium conversion. You still need a Tentative Parcel Map or Tentative Map; the County posts an ADU condo checklist on its ADU page. Our ADU real estate guide covers how ADUs affect value.

ADU permits and rents in the unincorporated county

ADUs are now a large share of new homes here. The County's 2025 housing progress report counts 403 ADUs among 932 homes permitted in the unincorporated area in 2025, about 43%. The same report averaged Zillow ADU listings from November 2025 to January 2026 for the unincorporated county:

Rents in the cities ran higher at most sizes, and the report found a price premium around Rancho Santa Fe. See ADU rental income in San Diego for the math, and compare verified ADU builders in San Diego County.

Common questions