Junior ADUs in California: rules, size and renting

What a junior ADU is

A junior ADU is a small home created inside an existing or proposed single-family house. HCD's ADU Handbook (March 2026) defines it as no more than 500 sq ft of interior livable space, contained entirely within a single-family residence, with either its own bathroom or a bathroom shared with the rest of the house (Government Code section 66333).

The rules that decide most projects:

• One per property. Only one junior ADU is allowed on a property zoned for single-family homes with an existing or proposed single-family house. If there are several detached houses on one property, there can still only be one.
• Inside the walls. It can be created anywhere within the house, including an attached garage. It cannot go in a detached garage or other accessory building.
• Single-family only. A duplex on one property counts as multifamily and does not qualify.

A junior ADU is reviewed the same way as other ADUs: by the city or county against fixed standards, without a hearing.

The 500 sq ft limit and what counts

Since January 1, 2026, under SB 543, every square-foot limit in ADU law means interior livable space. The Handbook's glossary describes it as space for living, sleeping, eating, cooking or sanitation, including bedrooms, bathrooms, kitchens, laundry rooms, closets and interior stairs. Wall thickness, garages and decks do not count.

Two points catch people out:

• No 150 sq ft extra. A conversion ADU can add up to 150 sq ft for an entry. A junior ADU cannot.
• Local minimums. Some cities set a minimum as well. The City of San Diego, for example, allows a junior ADU from 150 to 500 sq ft.

If you want more than 500 sq ft, the same space can usually become a conversion ADU instead, which has no 500 sq ft limit and no owner-occupancy rule.

Kitchen, bathroom and entrance

A junior ADU needs an efficiency kitchen. HCD's Handbook describes it as a cooking facility with appliances, a food preparation counter and storage cabinets of reasonable size for the junior ADU. A city cannot be stricter than that definition. It cannot require specific counter or cabinet sizes, particular electrical or gas connections, or certain appliance types, and a 220V circuit is not required.

The bathroom can be its own or shared with the main house. That choice matters more than any other, because it decides the owner-occupancy rule below.

State law does not require an inside door between the junior ADU and the house. But if the junior ADU shares a bathroom, it must have an interior entrance to the main living area. Many cities also require a separate exterior entrance; the City of San Diego requires its own exterior entry, for example.

Owner occupancy after AB 1154

Before 2026, the owner had to live on the property whenever there was a junior ADU. AB 1154 changed that from January 1, 2026:

• Own bathroom. No owner-occupancy requirement, and a recorded covenant cannot add one.
• Shared bathroom. The owner must live in either the rest of the house or the junior ADU. The Handbook notes an exception when the owner is a government agency, land trust or housing organization.

Some local codes still carry the older rule. The City of San Diego's municipal code, the County of Orange's zoning code and the City of Santa Ana's ADU pages all describe owner occupancy as always required. State law wins where they conflict, and the City of San Diego's own Information Bulletin 400 (January 2026) already reflects the new rule. Cities may still require a recorded junior ADU agreement; San Diego does, before the permit is issued.

Owner occupancy has never applied to regular ADUs. State law does not allow it to be required for one (Government Code section 66315).

Renting a junior ADU

A junior ADU can be rented. Since January 1, 2026, it cannot be used as a short-term rental, and a rental has to be longer than 30 days.

A junior ADU stays part of the house. In the City of San Diego, the code says plainly that a junior ADU cannot be sold separately from the home. Treat it as part of the house when you plan a sale, and ask the city if you have questions about your property.

Parking

No parking can be required for a junior ADU. It is part of the existing home, and state law does not allow parking to be required for an ADU inside the existing home. HCD's Handbook adds that this holds even when the junior ADU is converted from an attached garage.

When a garage is converted, state law also says the lost parking does not have to be replaced. One local exception: in the City of San Diego's Beach Impact Area, outside a transit priority area, a garage converted for a junior ADU must be replaced.

Fees and permits

The fee rules favor junior ADUs:

• Impact fees. None on a junior ADU of 500 sq ft or less.
• School fees. None on an ADU or junior ADU of 500 sq ft or less.
• Utility connections. No new connection fee or capacity charge for a junior ADU, unless it is built with a new single-family home.

Plan check and permit fees still apply and are set by each city. On timing, the city has 15 business days to say whether the application is complete, and 60 days from a complete application to approve or deny; if it misses that, the application is deemed approved.

Fire rules are set locally. The City of San Diego's bulletin says a junior ADU needs sprinklers if the main home has them, may need fire-rated shared walls, and does not need solar panels.

A junior ADU alongside other ADUs

Since January 1, 2026, a single-family property can combine a junior ADU, a conversion ADU and a detached ADU under the ministerial path. So adding a junior ADU in a spare bedroom or attached garage does not use up your right to a backyard ADU later. See converting a garage into an ADU for how a conversion compares.

Where ADUCheck applies local rules

ADUCheck reports use each city's own rules in the City of Los Angeles and the City of San Diego. Elsewhere in Los Angeles, Orange and San Diego counties, reports use the state rules above and are labelled State rules only, because a city may be more generous but not stricter.

A guide is a starting point, not a permit. Enter your address at aducheck.com and the report shows what fits on your property, including whether a junior ADU does, with the source and date for each fact.