What is an ADU?
An ADU, or accessory dwelling unit, is a smaller secondary home on the same lot as a primary residence. ADUs provide complete independent living facilities, including a kitchen, bathroom, and sleeping area.
The California Energy Commission describes an ADU as an attached or detached residential dwelling unit with complete independent living facilities on a lot with a proposed or existing primary residence. This definition emphasizes that an ADU is fully self-contained and not dependent on the primary home for basic utilities or daily living needs.
How California defines an ADU
California Government Code Section 65852.2 is the main statewide statute governing ADUs. This law creates a statewide framework that applies to all cities and counties in California, meaning local agencies must follow this baseline regardless of their own preferences.
The state also publishes the ADU Handbook through the Housing and Community Development Department (HCD). This handbook provides official guidance on how to interpret and apply the statewide law.
Under state law, an ADU must have independent living facilities. That means the unit must include a kitchen, a bathroom, and a sleeping area that function completely separately from the primary residence.
Common ADU types
The most common ADU paths are:
Detached ADU
A separate structure, often in the backyard. It stands completely apart from the primary home.
Attached ADU
A new unit connected to the main home, such as an addition or new wing.
Conversion ADU
An ADU created by converting existing interior space or accessory space (like a bonus room or storage area).
Garage conversion ADU
A specific conversion path where a garage is converted into a dwelling unit. This is often the first path homeowners explore because garages may already have water, electrical, and other utility connections nearby.
JADU
A junior accessory dwelling unit, which is related to but different from a full ADU. As of January 1, 2026, a JADU is limited to a maximum of 500 square feet of interior livable space.
ADU vs JADU
While related, ADUs and JADUs have important differences:
Size limits
ADUs can be larger and have fewer strict size restrictions. JADUs are limited to 500 square feet of interior livable space as of January 1, 2026.
Location
ADUs can be detached, attached, or created through conversion. JADUs must be created within the existing walls of the primary home—you cannot expand the primary home footprint to create a JADU.
Kitchen requirements
ADUs require a full, independent kitchen. JADUs may have an efficiency kitchen or kitchenette with reduced counter and storage space.
Separate entrance
Both ADUs and JADUs require a separate entrance from the primary home. For JADUs, this entrance must be accessible from the exterior.
Owner occupancy
JADUs have specific owner-occupancy requirements that do not apply to all ADU types. At least one of the units (the primary home or the JADU) must be owner-occupied.
For detailed information about JADUs, see the JADU Guide.
Why homeowners build ADUs
Most homeowners look into ADUs for one or more of these reasons:
- Housing for parents, adult children, or extended family members
- Rental income from a second unit on the property
- Flexible backyard or sideyard living space
- Better use of existing structures like garages or accessory buildings
- Long-term property value and housing flexibility
- Multigenerational living arrangements
California's ADU policies reflect a statewide housing effort to support smaller and more flexible housing options that can meet diverse family and community needs.
What ADUs are not
Understanding what does not qualify as an ADU is also helpful:
- A guest house or accessory structure without a kitchen—it must have complete independent living facilities
- A commercial space or office—ADUs are residential by definition
- An RV, mobile home, or temporary structure—ADUs must be permanent, fixed structures
- A separate legal lot—an ADU remains on the same parcel as the primary residence
Where to start
A clear first-step checklist for most homeowners:
- 1Identify which city or county has jurisdiction over your property
- 2Learn the statewide ADU basics (what this page covers)
- 3Check your local planning department's ADU page or guidelines
- 4Define your goal for the ADU—family housing, rental income, or flexible space
- 5Compare which ADU type makes the most sense for your site and budget
- 6Understand the permit timing and code cycle that will apply to your project
Recommended next steps
From here, most readers should explore one of these pages next:
This page is for general educational purposes only. California ADU law, city implementation, and code requirements can change. Always verify project-specific requirements with the relevant planning department, building department, and qualified professionals.