What are the ADU rules in California?

An ADU, short for accessory dwelling unit, is a second home on a single-family lot. It can be detached, attached to the main house, or built by converting an existing garage. A JADU, or junior accessory dwelling unit, is smaller and sits entirely inside the walls of the existing house, usually a converted bedroom with its own entrance and a small kitchen. California has spent years rewriting its housing laws to make them easier to build.

The state caps a detached ADU at 1,200 square feet, and an attached ADU at half the square footage of the primary home. A JADU tops out at 500 square feet and has to be built within the existing house, not added on. State law also sets a floor: whatever a city’s zoning says, it has to allow an ADU of at least 800 square feet with no more than a 4-foot side and rear setback, and it cannot cap a one-bedroom ADU under 850 square feet, or a multi-bedroom one under 1,000.

What state law requires cities to allow

Cities used to be the main obstacle to building an ADU. That has largely changed. Government Code sections 66314 and following, which took over from the older section 65852.2 when the Legislature reorganized this part of the code, spell out the objective standards a city can apply. Anything outside that list, a city cannot use to block or shrink your project. Review has to be ministerial: a planner checks the application against the objective standards and approves it, with no hearing and no discretionary vote.

That does not mean every lot qualifies or every design sails through. Cities still control water and sewer capacity, design review, and where on the lot the unit can sit. What they lost is the ability to say no just because they do not want more density on the block.

Do you have to live on the property

For a stand-alone ADU, no. State law bars cities from adding an owner-occupancy requirement on top of the standards already listed for ADUs. You can build a detached ADU and rent out either unit, or both, as long as neither is rented for less than 30 days.

A JADU is different. Because it is carved out of your existing house and often shares a bathroom with it, the law requires the owner to live in either the main house or the JADU, unless it has its own separate sanitation or the property is owned by a public agency, land trust, or housing nonprofit.

Setbacks, height, and parking

A converted structure, like an existing garage turned into an ADU, needs no new setback at all. A new detached ADU needs no more than a 4-foot side and rear setback. Height runs 16 feet for a standard detached unit, 18 feet within a half mile of a major transit stop, with 2 more feet allowed to match the primary home’s roof pitch. An attached ADU can go up to 25 feet or the height the city allows for the main house, whichever is lower, capped at two stories.

Parking is where most of the old friction lived, and it has mostly cleared away. A city can require at most one space per ADU or per bedroom, whichever is fewer, and has to waive it entirely for a unit within a half mile of transit, in a historic district, or created by converting part of the existing house or garage.

How fast the city has to move

Once you submit a complete ADU application, the city has 60 days to approve or deny it. Miss that window and the application is deemed approved by operation of law. Before that clock starts, the city has 15 business days to say if your application is incomplete, so it cannot just sit on a file. If a city denies an application, it has to hand you a specific written list of what was deficient, not a vague rejection.

Impact fees are separate from the approval timeline, and there is no single statewide number to point to. Water, sewer, and school fees often come from different agencies than the one issuing your permit, and how they treat a small ADU varies by city. Ask your building department for the actual fee schedule for an ADU your size before you budget, rather than assuming it will track a new single-family permit.

What an ADU does to your property taxes

Building an ADU triggers a reassessment, but only of the new construction. Under the rules the Board of Equalization applies, an addition like an ADU gets its own new base year value, based on what it added to the property’s market value. The rest of your home, land included, keeps its existing Proposition 13 base year value and is not reappraised. Your tax bill goes up by the ADU’s added value, not by a reassessment of the whole property.

SB 9 is a different law

If you have heard about splitting a single-family lot in two, or adding a duplex, that is Senate Bill 9, not the ADU rules above. It lets an owner in many single-family zones split a lot and add units under its own ministerial process, with its own owner-occupancy affidavit. The two laws are not the same tool, and SB 9 deserves its own conversation with your city before you plan around it.

Buying or selling: check before you count on the income

An ADU or converted garage that was never permitted is one of the more common surprises to turn up during an inspection. It might look finished and lived-in and even be generating rent right now, but with no permit history, a lender’s appraiser can refuse to count that square footage or that rental income at all, at the worst possible point in escrow.

If you are selling a home with an ADU or garage conversion, disclose it the same way you would any other known condition, and say whether it was permitted. If you are buying and the seller is counting rental income toward what the property can support, verify the permit before you rely on that number, especially if you are financing with a DSCR loan that qualifies you on the property’s income instead of your own. A pre-listing inspection or a buyer’s inspection will usually flag an unpermitted conversion, but the disclosure obligation exists either way.

The short version

An ADU can be up to 1,200 square feet detached, or half your home’s size if attached, and a JADU tops out at 500 square feet inside the existing house. State law forces cities to approve a compliant application within 60 days, caps what they can require on setbacks, height, and parking, and does not require you to live on-site unless you built a JADU with shared facilities. None of that guarantees the income pencils out, so verify permits, financing, and rent numbers before you count on any of it.

If you are weighing whether an ADU adds enough value to be worth it on your property, start with the home value tool. It is a starting point before a conversation about your specific home.