A junior ADU is a unit of up to 500 square feet created inside an existing single-family house, with its own entrance and a compact kitchen. It can share a bathroom with the main house. You can have one per lot, and it can be built alongside a full ADU.
How it differs from a full ADU
A junior ADU has to be inside the walls of the existing house, which includes an attached garage. It tops out at 500 square feet of interior space. It needs its own exterior entrance, and if it does not have its own bathroom, an interior door connecting it to the house. A full ADU can be detached, can run larger, and has no owner-occupancy requirement at all.
The owner-occupancy question
For a regular ADU, cities cannot require you to live on the property. For a junior ADU the rule is narrower: owner-occupancy can only be required where the junior unit shares a bathroom with the main house. A 2025 law removed it for junior units with their own bathroom.
Rentals of a junior ADU have to be longer than 30 days, so short-term rental is out.
Why build one
Cost. A junior ADU reuses a bedroom and part of a hallway, so you are mostly paying for a kitchenette, a separate entrance, a fire-rated door and some electrical work. A lot of owners build a junior ADU and a detached ADU at the same time and end up with two rentals.
Los Angeles County figures, reviewed 2026-10-29 by America General Contractors, California contractor licence CSLB 1083262. Planning ranges, not a quote, and not legal advice. We confirm the current rules for your parcel before anything gets drawn.
