An accessory dwelling unit (ADU) is a second, independent home on a lot with a single-family or multifamily house. It has its own entrance, kitchen and bathroom, and it can be a backyard cottage, an addition attached to the house, or a converted garage. You may also hear ADUs called granny flats, in-law units, backyard cottages or casitas.
Types of ADUs
- Detached ADU: a separate building, usually in the backyard.
- Attached ADU: an addition that shares a wall with the main house but has its own entrance.
- Garage conversion: an existing garage turned into a legal living unit. See garage conversion ADU requirements.
- Junior ADU (JADU): up to 500 square feet inside the existing house. See what is a JADU.
What California law guarantees
California’s ADU law (Government Code Section 66314 and following) applies in every city and county:
- Cities must approve or deny a complete ADU application within 60 days, without a public hearing.
- Cities must allow at least an 800 square foot ADU with 4-foot side and rear setbacks, and at least 16 feet of height for a detached ADU.
- No replacement parking can be required when a garage is converted to an ADU.
- Impact fees cannot be charged on ADUs under 750 square feet.
- Owner occupancy cannot be required for an ADU.
Each city then sets its own rules on top of that floor, such as maximum size, height, design standards and tree protection.
What can you do with an ADU?
- House aging parents or adult children close by.
- Rent it for added income, for terms longer than 30 days.
- Create a home office, studio or guest house.
- Live in the ADU and rent the main house.
In most cities an ADU cannot be sold separately from the main house.
How an ADU gets built
- Site assessment: setbacks, trees, slope, sewer, electrical service and fire access.
- Design, structural and energy documents.
- City permit, usually a ministerial building permit.
- Utility coordination, including whether the ADU shares the main electric meter.
- Construction and inspections.
- Final inspection and certificate of occupancy.
Read our step-by-step guide on how to build an ADU in your backyard, see the rules for your city in our Bay Area ADU guide, or follow a real 1,200 sq ft ADU we are building in Los Altos.
