State law sets the ground rules for accessory dwelling units, and every city and county has to follow them. Local ordinances fill in the details and can be less strict than the state, so always check with your city or county too. The state’s own guide is the HCD ADU Handbook.
ADU or JADU?
- ADU: “an attached or a detached residential dwelling unit that provides complete, independent living facilities for one or more persons,” on a lot with a proposed or existing home (HCD Handbook). It can be a new backyard building, an addition, or a converted garage.
- JADU (junior ADU): a unit of “no more than 500 square feet of interior livable space” that is “contained entirely within a single-family residence” (Gov. Code §66313). The Handbook counts an attached garage as part of the house for this.
How big can it be?
State law caps a detached ADU at 1,200 square feet (Gov. Code §66314), but a city or county can allow more: “maximum unit sizes can exceed 1,200 square feet for ADUs through the adoption of a less stringent local ADU ordinance” (HCD Handbook).
Going the other way, a local size limit can’t be set below 850 square feet for a studio or one-bedroom ADU, or 1,000 square feet for an ADU with more than one bedroom (Gov. Code §66321).
Setbacks and height
| Rule | What the state allows |
|---|---|
| Side and rear setbacks | No more than 4 feet can be required |
| Detached ADU height | At least 16 feet |
| Detached, near transit | 18 feet, with an allowance for roof pitch, within a half-mile of a major transit stop or high-quality transit corridor |
| Attached ADU height | 25 feet, or the main home’s height limit if lower |
Source: HCD Handbook. These are the minimums a local agency must allow. In the Coastal Zone, extra setbacks “may be required” if the Local Coastal Program calls for them.
Parking and garage conversions
A city or county can’t require parking for an ADU in several situations, including when the ADU is within one-half mile walking distance of public transit (Gov. Code §66322). The Handbook lists six exemptions in all.
Converting a garage? When a garage, carport or parking space is converted to an ADU or demolished to build one, “the local agency shall not require that those offstreet parking spaces be replaced” (Gov. Code §66314).
How long the permit takes
Once your application is complete, and there’s already a home on the lot, the city or county has 60 days to approve or deny it. “If the local agency has not approved or denied the completed application within 60 days, the application shall be deemed approved” (Gov. Code §66317).
In the Coastal Zone, ADU law still applies, but it doesn’t lessen the Coastal Act’s resource protections. A coastal development permit for an ADU must also be approved or denied within 60 days of a complete application when there’s an existing home, without a public hearing (HCD Handbook).
Impact fees
An ADU with 750 square feet or less of interior living space is exempt from impact fees. Larger ADUs can be charged impact fees in proportion to their size compared with the main home (Gov. Code §66311.5; HCD Handbook). Other costs, such as plan check and utility connections, vary by city, county and utility.
Renting, selling and living on site
- Renting: an ADU “may be rented separate from the primary residence” (Gov. Code §66314).
- Selling separately: generally not allowed. A city or county can adopt its own ordinance allowing the home and ADU to be sold separately as condominiums, under specific state requirements (HCD Handbook).
- Owner-occupancy: a local agency generally can’t require the owner to live on the property because of an ADU. JADUs are different: owner-occupancy is required if the JADU shares sanitation facilities (a bathroom) with the main house (HCD Handbook).
Local planning pages
- Santa Barbara County: Accessory Dwelling Units
- San Luis Obispo County: Accessory Dwelling Units
- City of Santa Barbara: ADU project guidance
- In Santa Maria, the City of San Luis Obispo and other cities, check with that city’s planning or building department.
What an ADU costs
Cost depends mostly on size, whether it’s detached, attached or a conversion, site conditions such as slope and how far utilities have to run, and finishes. Our ADU page walks through the steps and the main cost factors.
Sources
- HCD Accessory Dwelling Unit Handbook (March 2026)
- HCD ADU handbook page
- Government Code §66313 (definitions)
- Government Code §66314 (standards)
- Government Code §66317 (permit timing)
- Government Code §66321 (minimum sizes)
- Government Code §66322 (parking)
- Government Code §66311.5 (impact fees)
- Santa Barbara County: Accessory Dwelling Units
- San Luis Obispo County: Accessory Dwelling Units
- City of Santa Barbara: ADU project guidance