What is an ADU? Types, uses and the San Diego process

What counts as an ADU

California law defines an accessory dwelling unit as an attached or detached home with “complete independent living facilities”: permanent provisions for living, sleeping, eating, cooking and sanitation, on a lot with a main house. Efficiency units and manufactured homes can qualify.

The state housing department notes ADUs go by many names, including granny flats, in-law units, backyard cottages and secondary units.

The four types

ADU types
Type	What it is	Good to know
Detached	A new, stand-alone building in the yard	Most privacy; most site work and cost
Attached	An addition joined to the house	Can share a wall and utilities with the house
Conversion	A garage, basement or other existing space turned into a home	No size limit and no setbacks for existing space
Junior ADU	Up to 500 sq ft carved out of the house itself	Can share a bathroom with the house; one per lot

Definitions and limits: state law and the state housing department. The County of San Diego also shows units above a garage and on an upper or lower floor of the house. For what each costs, see what an ADU costs in San Diego.

What people use them for

A statewide survey of ADU owners found they valued them for housing family and for extra rental income. Only 8% of ADUs were used as short-term rentals. That's partly because most cities, San Diego included, require ADU rentals to be longer than 30 days. The ADU laws guide explains the rental rules.

Adding an ADU doesn't reassess your existing home. Only the value of the new construction is added to your property tax assessment.

Who issues your permit

San Diego County has 18 cities, and each runs its own ADU permits under the same state law. If your home is in the City of San Diego, you apply to its Development Services Department. If it's outside any city, in an unincorporated community, the County handles it. Some city ADU pages:

Free plans from the County. The County of San Diego publishes free ADU plans, about 85% complete, from a 600 sq ft one-bedroom to a 1,200 sq ft three-bedroom. Several cities, including San Diego, accept them.

From idea to move-in

Check what fits. Lot size, slope, utilities, fire zone and coastal zone decide what you can build and what it costs.
Design. A designer or design-build contractor draws the plans, with structural and energy calculations. A pre-approved plan shortens this.
Apply. In the City of San Diego, ADU applications are submitted online through the Development Services application portal.
Plan review. Under state law, the city has 15 business days to confirm your application is complete and 60 days to approve or deny it.
Build and inspect. Each stage is inspected before it's covered up.
Final inspection. In San Diego, passing the final inspection is the approval to move in.

Before you start, read how to hire an ADU contractor.

How long it takes

The San Diego Housing Commission put a typical ADU at about 8 to 12 months from idea to finished construction; its own pilot units took 10 to 26 months. Builders estimate 3 to 9 months for design and permits and 5 to 7 months to build. Slopes, coastal permits and redesigns add time.

How many are being built

California permitted 30,354 ADUs in 2024, up from 1,336 in 2016. ADUs were more than a quarter of all homes permitted in the state. The City of San Diego permitted more than 2,285 ADUs in 2024, about 26% of all new homes, led by Clairemont Mesa, North Park and Southeastern San Diego. Across the county, ADU permits rose from 1,150 in 2020 to 3,991 in 2024.

Common questions

What's the difference between an ADU and a junior ADU?

A junior ADU is 500 square feet or less and sits inside the main house, and can share its bathroom. An ADU can be detached, attached or converted, is usually larger, and is fully self-contained.

Do I need a permit to build an ADU in San Diego?

Yes. Every ADU needs a building permit. The City of San Diego handles permits inside city limits; the County handles unincorporated areas; other cities run their own.

How long does it take to build an ADU?

Usually 8 to 12 months from idea to move-in, and often longer. Design and permits can take 3 to 9 months and construction 5 to 7 months.

Sources

California Legislative InformationGovernment Code § 66313 (definitions)
California Department of Housing and Community DevelopmentAccessory Dwelling Units
California Department of Housing and Community DevelopmentAccessory Dwelling Unit Handbook (March 2026)
County of San Diego Planning & Development ServicesAccessory Dwelling Units
County of San Diego Planning & Development ServicesPre-approved ADU plans
City of San Diego Development ServicesInformation Bulletin 400: Accessory and Junior Accessory Dwelling Units
City of San Diego Development ServicesInspection types
City of San Diego Planning Department2025 Annual Report on Homes
UC San Diego Center on Housing Policy and DevelopmentADU production in the San Diego region
San Diego Housing CommissionADU Pilot Program Report (October 2021)
Snap ADUHow long does it take to build an ADU in San Diego?
UC Berkeley Terner Center for Housing InnovationA new survey of ADU owners in California (2021)
San Diego County AssessorReassessments
California Legislative InformationGovernment Code § 66317 (permit timelines)

General information, not legal or financial advice. Rules and fees change; confirm with your city's permit office before you rely on them.