California has steadily expanded homeowners' right to build ADUs. Here is a plain-English summary of the current state rules and where to verify them.
Accessory Dwelling Units (ADUs) are one of the few places where California state law directly overrides local zoning to make it easier to add housing. If you own a single-family or multi-family lot in California, state law generally guarantees you the right to build at least one ADU and one Junior ADU, subject to basic objective standards.
What state law guarantees
Under the California Department of Housing and Community Development (HCD), every city and county must allow ADUs and Junior ADUs on lots zoned for residential or mixed use. Local agencies cannot impose minimum lot sizes for most ADUs, cannot require replacement parking when converting a garage, and must act on a complete ADU application within 60 days.
HCD publishes the authoritative ADU Handbook that lays out maximum allowable sizes, setback rules (generally 4 feet side and rear), and height limits. As of the most recent handbook update, detached new-construction ADUs may be built up to 1,200 square feet and up to 16 to 18 feet tall depending on the lot context, and many of those standards cannot be reduced by local ordinance.
What changed recently
A series of bills passed since 2019 has progressively loosened restrictions: owner-occupancy requirements were suspended for ADUs permitted through 2025, impact fees were eliminated for ADUs under 750 square feet, and cities were required to approve pre-approved ADU plan sets for faster permitting. More recent legislation expanded the right to sell an ADU separately from the primary home in certain nonprofit and affordable-housing contexts.
Because the legislature adjusts these rules almost every session, the current text of the Government Code sections governing ADUs (§§ 66314–66333) is the definitive source. You can read the live, updated statute on the California Legislative Information site.
What this means for your project
In practice, the state floor means most San Diego-area homeowners can count on being allowed to build a detached ADU of meaningful size, even if their local zoning is restrictive. Our team at Casita handles the research for your specific parcel so you know exactly what you can build before you commit. See how we structure that in our process overview at casitaadu.com/process.
Official sources
California HCD ADU page: https://www.hcd.ca.gov/planning-and-community-development/accessory-dwelling-units California HCD ADU Handbook (PDF): https://www.hcd.ca.gov/planning-and-community-development/accessory-dwelling-units/docs/adu_handbook.pdf California Government Code §§ 66314–66333: https://leginfo.legislature.ca.gov/faces/codes_displayText.xhtml?lawCode=GOV&division=&title=7.&part=&chapter=13.&article= Casita ADU process: https://www.casitaadu.com/process