ADU Definition in California: Size, Setbacks, and Permits

In California, an accessory dwelling unit (ADU) is defined by state law as an attached or detached residential unit on the same lot as an existing or proposed primary home, with complete independent living facilities including permanent provisions for sleeping, cooking, eating, and sanitation.¹ That statutory definition gives ADUs a protected legal status that overrides most local zoning restrictions on size, parking, setbacks, and occupancy.

What Makes a Unit an ADU

The legal test is self-sufficiency. The unit has to work as a complete, independent home for one or more people, with its own kitchen, bathroom, sleeping area, and living space. The form it takes is flexible. An ADU can be a brand-new detached structure, an addition attached to the primary house, or a conversion of existing space such as a garage or basement. The definition also covers efficiency units and manufactured homes.¹

Junior ADUs Are a Separate Category

A junior accessory dwelling unit (JADU) is a smaller subcategory governed by its own statute. A JADU is capped at 500 square feet and must sit entirely within the walls of an existing or proposed single-family home, including enclosed spaces like an attached garage. Unlike a full ADU, a JADU can share bathroom facilities with the primary residence. It must include an efficiency kitchen with cooking appliances and a food preparation counter with storage cabinets sized appropriately for the unit.²

How Many ADUs One Lot Can Have

State law sets a floor that every local agency has to allow. On a single-family lot, the local agency must permit at least one ADU converted from existing space, one newly constructed detached ADU, and one JADU.³ A homeowner with a single-family house can potentially have both a full ADU and a JADU on the same property.

Multifamily lots get larger allowances. Local agencies must permit at least one ADU converted from existing non-livable space (or up to 25 percent of the number of existing multifamily units) plus two detached ADUs. For lots with an existing multifamily building, up to eight detached ADUs are authorized, provided the total does not exceed the number of existing units on the lot.³

Size Limits

State law draws clear lines around how large or small an ADU can be, and the limits differ by type.

A detached ADU can go up to 1,200 square feet.¹
An attached ADU can be up to 50 percent of the existing primary dwelling’s floor area, but local agencies cannot restrict the size below 800 square feet even when 50 percent of the primary home comes out smaller than that.³
A JADU tops out at 500 square feet.²
The minimum for any ADU is 150 square feet, matching the state’s efficiency unit standard.³

Local agencies can adopt their own size ordinances. What they cannot do is set a maximum below 850 square feet for a studio or one-bedroom ADU, or below 1,000 square feet for an ADU with two or more bedrooms. As of 2026, these size limits are measured by interior livable space rather than total floor area.³

Height and Setback Rules

Height depends on the type of ADU and where the lot sits. For a detached ADU on a single-family lot, the baseline maximum is 16 feet. That rises to 18 feet when the ADU is within a half-mile walking distance of a major transit stop, or when it sits on a lot with an existing multifamily or multistory building. An additional two feet is allowed to match the roof pitch of the primary home.³

Attached ADUs get more vertical room. A local agency cannot impose a height limit below 25 feet on an attached ADU, though it does not have to allow more than two stories. If the primary dwelling already has a lower height limit, that limit controls.³

One detail trips people up. If the height allowances permit a two-story detached ADU that complies with building code, the local agency cannot deny it because the underlying zoning limits the primary dwelling to one story.³

Side and rear setbacks for an ADU cannot exceed four feet. No setback is required at all when the ADU is converted from existing living space or an accessory structure, or when a new structure is built in the same location and dimensions as a structure being replaced. Local agencies can apply front yard setbacks, but those setbacks cannot make it impossible to build an ADU of at least 800 square feet on the property.³

Parking

Parking is where California’s ADU law is most generous. A local agency cannot require more than one parking space per ADU or per bedroom, whichever is less, and those spaces can be provided as tandem parking on a driveway. Guest parking cannot be required. When a garage, carport, or other parking structure is demolished or converted to build an ADU, the local agency cannot demand those lost spaces be replaced.³

Many ADUs qualify for a full parking exemption, which means zero spaces required. The exemption applies when the ADU is:

Within a half-mile walking distance of public transit
Inside an architecturally and historically significant district
Part of the existing primary residence or an accessory structure
On a property where on-street parking permits are required but not offered to the ADU occupant
Within one block of a car-share vehicle

JADUs are entirely exempt from parking requirements, even when converted from an attached garage.³

Owner Occupancy, Rentals, and Sale

Local agencies cannot impose an owner-occupancy requirement on a standard ADU.³ You can rent out both the primary dwelling and the ADU and live somewhere else entirely.

JADUs work differently. Traditionally, the owner had to live in either the JADU or the remaining portion of the primary home. Starting January 1, 2026, the rule narrows: owner-occupancy is required only when the JADU shares sanitation facilities with the primary structure. A JADU with its own bathroom does not trigger the requirement. Governmental agencies, land trusts, and housing organizations that own the property are also exempt.³

An ADU can be rented separately from the primary residence.¹ Local jurisdictions can require a minimum rental term of 30 days or longer, which effectively bans using an ADU as a vacation rental or short-term listing.

The general rule is that an ADU cannot be sold or conveyed separately from the primary residence. AB 1033, which took effect in 2023, opened a narrow path: a local agency can adopt an ordinance allowing the primary dwelling and ADU to be sold as separate condominiums if specific requirements under Government Code Section 66342 are met. Not every city has adopted such an ordinance, so this option depends on local action. JADUs remain permanently prohibited from separate sale.³

Permits, Fees, and Utilities

State law requires local agencies to handle ADU applications through ministerial approval. Review is limited to whether the project meets objective building and zoning standards. No public hearings, no subjective design reviews, no neighborhood compatibility arguments. A local agency must approve or deny a completed ADU application within 60 days.³ If a local government has not adopted an ADU ordinance that complies with state law, the state standards apply directly.

ADUs with 750 square feet or less of interior livable space are exempt from development impact fees. JADUs with 500 square feet or less are similarly exempt. For ADUs larger than 750 square feet, impact fees must be charged proportionally relative to the square footage of the primary dwelling, not at the flat rate a new standalone home would face.³

When an ADU is created from existing space in a primary dwelling or accessory structure, the local agency cannot require a new or separate utility connection, unless the ADU is being built at the same time as a new single-family home.³ Detached new-construction ADUs may need their own hookups, which typically means additional connection fees from the utility provider.

Property Tax Treatment

Building an ADU does not trigger a reassessment of the entire property. California assesses only the value of the new improvement, leaving the existing home’s assessed value and its Proposition 13 protection intact. The additional property tax is based on the assessed value of the ADU itself. For Proposition 19 purposes, an ADU is considered part of the primary residence, so a homeowner who lives in either the main house or the ADU on the lot can still qualify for a base-year value transfer when they move.

A Note on the 2024 Statutory Renumbering

Anyone researching the law will run into two sets of section numbers. The original ADU statute was Government Code Section 65852.2, and the JADU statute was Section 65852.22. In 2024, these were renumbered to Sections 66310 through 66342.⁴ The substance carried over. Local ordinances, permit applications, and official guidance now use the new numbering, but older materials citing 65852.2 point to the same framework.

1
California Legislative Information. California Government Code 65852.2 – Accessory Dwelling Units
2
California Legislative Information. California Government Code 65852.22 – Junior Accessory Dwelling Units
3
California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
4
ABAG. ADU State Laws Summary and Checklist