ADU height limit in California: 16, 18 and 25 feet explained

ADU height limit in California

Government Code section 66321(b)(4) lists the lowest height limits a city may apply to an ADU. A city can set a higher limit, but an ordinance that holds an ADU below these numbers can't be enforced against it.

The state also ties height to its guaranteed ADUs. Section 66323 requires every city to approve one new detached ADU of up to 800 square feet with 4-foot side and rear setbacks on a single-family lot, at these height limits. Even a strict city must let you build that unit at 16 feet or more.

Cities don't have to allow an ADU taller than two stories. HCD's ADU handbook says a city must allow at least two stories when they fit under the applicable height limit and the building code. For the two-story question in depth, see our two-story ADU guide; for everything else in state law, see California ADU laws.

Who gets 18 or 20 feet

The higher detached limit depends on how far your lot is from transit. Section 66321 gives 18 feet to a detached ADU within one-half mile walking distance of a major transit stop or a high-quality transit corridor, as Public Resources Code section 21155 defines them, and lets you add up to 2 more feet so the ADU's roof pitch can match the main house. That's up to 20 feet.

The walking-distance point matters. Other transit-based housing rules are measured differently: HCD's guidance on the AB 2097 parking law says that law's half mile is generally measured in a straight line. Don't assume a transit map drawn for parking also answers the height question. HCD's handbook points to the California State Geoportal to check whether a lot is near a high-quality transit corridor.

Attached ADUs, conversions and JADUs

Attached ADUs

An ADU attached to the house gets the most room: at least 25 feet, or whatever the zone allows for the house if that's lower. That's why a second-story ADU over the house or an attached garage is often easier to permit than a tall detached one. Our attached vs detached ADU comparison covers the other trade-offs.

Conversions

A conversion keeps the height of the structure it's in. State law lets you convert an existing structure, or rebuild one in the same place and to the same dimensions, without new setbacks. If you raise the roof or add a story, the new part is treated as new construction and has to meet the height and setback rules for a new ADU. See the garage conversion ADU guide.

Junior ADUs

A junior ADU sits inside the existing single-family home, so it doesn't raise a separate height question. It's limited to 500 square feet of interior space. More in our junior ADU guide.

How ADU height is measured

State ADU law sets the numbers but doesn't define how to measure them, so cities use their own zoning definitions. That can swing the result by several feet on the same design. Some measure to the highest point of the roof. Others measure to the midpoint of a pitched roof or from a different grade line on a slope.

HCD has pushed back on stricter methods. In a 2024 review, HCD told Del Rey Oaks to align its ADU height measurement with the California Building Code, which defines building height as the vertical distance from the grade plane to the average height of the highest roof surface. Measured that way, a pitched roof can rise above the stated limit at the ridge.

Get the city's answer in writing or in its code before you lock in a roof design.

What fits in a 16 foot ADU

Sixteen feet is the number most homeowners hit, because it's the state floor and many cities stick close to it. Inside, the California Residential Code requires habitable rooms to have a ceiling height of at least 7 feet. The rest of the 16 feet goes to the floor system, the roof structure and the roof itself.

• One story fits easily. A single-story ADU at 16 feet can have a pitched roof and vaulted ceilings in the main room.
• A loft is tight. Usable sleeping lofts need headroom of their own, and at 16 feet the numbers rarely work for anything beyond storage.
• Two full stories usually don't fit. Two floors of 7-foot-plus ceilings, a floor system between them and a roof leave little room under 16 feet. Two-story ADUs depend on a local limit higher than the state minimum.
• Raised foundations cost height. On a slope or in a flood area, a crawlspace or raised floor eats into the limit, depending on how your city measures.

If you need more space and your city holds you to 16 feet, it's usually cheaper to build a larger single-story footprint than to fight for a height exception. Browse ADU floor plans for one-story layouts.

ADU height in San Diego

Local rules around San Diego go well past the state floor.

• City of San Diego. SDMC § 141.0302 holds ADUs to the overall maximum structure height of the base zone and any overlay zone, not a fixed 16 feet. On lots zoned for single-family but not multifamily, a detached ADU, or one attached to an existing accessory structure, can be up to two stories.
• Height changes the setback. In the city, an ADU 16 feet or lower needs no interior side or rear setback outside high fire zones. Go taller or to a second story, and a 4-foot setback (or the base zone setback if less) applies next to residential lots. See ADU setbacks.
• Coastal Zone. Within the city's Coastal Zone, roughly west of Interstate 5, no building may exceed 30 feet, and only city voters can change that. Our ADU coastal zone guide covers the permit side.
• Multiple ADUs. The city's ADU bulletin says multiple ADUs, including bonus ADUs, must stay within the zone's height, lot coverage and floor area ratio limits.
• Unincorporated county. The County of San Diego's ADU handout limits a detached ADU to 25 feet, except in multifamily complexes.

Other cities in the county set their own numbers within state law. If you're outside the City of San Diego, check your city's ADU ordinance, or ask one of our verified ADU builders in San Diego who works there.

Before you design

• Find your zone and overlays. The base zone, coastal or fire overlays and any historic district can each change the number. Fire areas bring their own rules; see building an ADU in a fire zone.
• Check transit distance if you want 18 or 20 feet, and measure it as a walk.
• Confirm how height is measured and from which grade.
• Check private rules. If you're in an HOA, its design review can still affect a tall ADU. See ADU HOA rules.
• Hire for the building you want. A taller ADU is new construction; look for a B, General Building contractor, and read how to hire an ADU contractor.

Height affects cost through framing, stairs and structure. See what an ADU costs in San Diego for budget ranges.

Common questions