How Big Can an ADU Be in Los Angeles? Size, Height, Setbacks

In Los Angeles, a detached ADU can be built up to 1,200 square feet, and an attached ADU is limited to 50 percent of the primary home’s living area with a guaranteed minimum of 850 square feet for a studio or one-bedroom and 1,000 square feet for a unit with two or more bedrooms. Those are the citywide ceilings. How big an ADU can actually be on your property in Los Angeles depends on your lot’s setbacks, height limits, floor area ratio, and whether you’re building new or converting something that already exists.¹

Detached and Attached Maximums

Detached ADUs in Los Angeles top out at 1,200 square feet of floor area. Lot-level rules on floor area ratio or residential floor area still apply, so on a smaller parcel the zoning math may cap you below 1,200 square feet before the citywide number matters.¹

Attached ADUs use a percentage rule. Floor area cannot exceed 50 percent of the existing primary dwelling. If the main house is 1,600 square feet, an attached ADU can run up to 800 square feet under that formula. The 50-percent cap has a floor built into it: you can always build an attached ADU of at least 850 square feet for a studio or one-bedroom, or 1,000 square feet if the unit has two or more bedrooms, regardless of what the percentage calculation produces.¹

The State Floor Your Lot Can’t Take Away

California Government Code Section 65852.2 sets a minimum no local rule can undercut. Cities cannot cap ADU size below 850 square feet for a studio or one-bedroom, or below 1,000 square feet for a unit with two or more bedrooms.²

State law also guarantees at least one detached ADU of up to 800 square feet, at 16 feet of height, with four-foot side and rear yard setbacks, on any qualifying lot. This is a backstop for properties where floor area ratio, lot coverage, or other zoning rules would otherwise block an ADU entirely.² Los Angeles applies the same rule: floor area limits on a lot cannot prevent an 800-square-foot detached ADU or an 850- or 1,000-square-foot attached ADU from being built.³

Junior ADUs and Stacking Units

A junior ADU is a smaller unit built inside the walls of an existing single-family home, typically a converted bedroom with a kitchenette and its own entrance. California law caps a junior ADU at 500 square feet, and that ceiling has no local override.⁴

The size trade-off comes with a payoff: a junior ADU can be built in addition to a standard ADU on the same single-family lot. A property in Los Angeles can host a detached ADU of up to 1,200 square feet plus a junior ADU of up to 500 square feet, on top of the primary home.² A junior ADU can share a bathroom with the main house, which keeps construction simpler.

Height Limits

Square footage is one dimension. Height controls whether that footage sits on one floor or two.

The 800-square-foot statewide exemption unit is capped at 16 feet. Within a half mile of a major transit stop, a detached ADU can reach 18 feet under state law, and up to 20 feet if the roof pitch matches the primary home.³

A detached ADU built under the city’s own ordinance can be up to two stories, subject to the zoning height limit for the zone. Attached ADUs under the ordinance are capped at the zoning height limit or 25 feet, whichever is lower. Many Los Angeles single-family zones run 28 to 36 feet, so 25 feet is the effective ceiling for most attached ADUs.³

Setbacks and Lot Coverage

The size on paper is not always the size on the ground. New ADU construction in Los Angeles requires a minimum four-foot setback from side and rear property lines. Front yard setbacks vary by zone, and ADUs generally cannot be placed in the front yard unless they convert a structure that already sits there.⁵

Lot coverage and floor area ratio further limit what fits. The combined footprint of all structures on your lot cannot exceed the percentage your zone allows. Even so, those limits cannot prevent an 800-square-foot detached ADU or an 850-square-foot attached ADU from being built. The guaranteed minimum sits above the FAR math.³

Garage and Existing Structure Conversions

Converting an existing garage or accessory structure has one significant size implication. Under state law and the city’s ordinance, if the ADU stays in the same location and dimensions as the existing structure, current setback requirements don’t apply.⁶ That matters in older LA neighborhoods where garages often sit on or near a property line.

The trade-off is that a conversion is bounded by the existing footprint. Many Los Angeles two-car garages run 380 to 440 square feet, enough for a studio or small one-bedroom but well below the 1,200-square-foot detached maximum. Expanding beyond the original walls brings the four-foot setback rule back into play. If you want a larger unit than the existing structure supports, new construction is usually the path.

Multi-Family Properties

For a property that already has a duplex, triplex, or apartment building, the rules shift. State law requires cities to allow at least one ADU created by converting existing non-livable space (storage rooms, basements, garages) inside the building, and up to 25 percent of the existing unit count. A 12-unit building can add up to three conversion ADUs from non-residential space.⁶

Detached ADUs are also allowed on multi-family lots. With an existing multi-family building, up to eight detached ADUs are permitted, and the number cannot exceed the existing unit count on the lot. With a proposed multi-family project, the cap is two detached ADUs. Each detached unit follows the same size rules as on a single-family lot, up to 1,200 square feet with four-foot side and rear setbacks.⁶

The 750-Square-Foot Fee Threshold

Size drives cost in one specific way worth planning around. Under state law, impact fees are waived entirely for any ADU with 750 square feet or less of interior livable space, and for any junior ADU of 500 square feet or less. Above 750 square feet, impact fees are charged proportionally based on the ADU’s size relative to the primary dwelling.⁶

A 749-square-foot design and an 800-square-foot design can land on opposite sides of that line, with sewer connection, school, and utility fees on one side and none on the other. If your plan sits near the threshold, running the numbers before finalizing the floor plan can be worth several thousand dollars.

Finding the Number for Your Specific Lot

Citywide maximums are a starting point, not an answer for a specific address. Zoning designation, lot dimensions, existing structures, and overlay districts like a Hillside Area or Historic Preservation Overlay Zone all pull the buildable size in different directions. Two properties on the same block can have very different ADU potential.

Start with your zoning through the LA City Planning website or ZIMAS, which shows your zone, height district, and any overlays. The Los Angeles Department of Building and Safety can then confirm what a given lot allows. Getting that read before hiring an architect saves rework at plan check. LADBS also runs a Standard Plan Program with pre-approved ADU designs, which shortens permitting at the cost of some design flexibility, and plan check on custom designs typically runs two to four months.

1
American Legal Publishing. Los Angeles Municipal Code SEC 12.22 – Exceptions
2
California Legislative Information. California Code Government Code 65852.2
3
Los Angeles Department of City Planning. ZA Memorandum No 143
4
California Legislative Information. California Code Government Code 65852.22
5
City of Los Angeles Department of City Planning. City of Los Angeles Accessory Dwelling Unit Memorandum
6
California Department of Housing and Community Development. Accessory Dwelling Unit Handbook