What you can build

Garage apartments: building an ADU above a garage

Checked against state laws and official agency sources · Last reviewed October 10, 2026

Garage apartment or garage conversion?

A garage conversion creates the ADU inside the garage's existing walls. The car space becomes living space. Our garage conversion guide covers that project.

A garage apartment puts the ADU above the garage, so the garage can stay a garage. You can build a new two-story building with parking below and the unit above, or add a second story on top of an existing garage.

Cities write rules for this second case. Burbank, California, says that when an ADU is built on top of a garage, "the ADU cannot touch grade level, except through support posts or stair access." The bottom of the ADU's finished floor must be above the top of the garage walls. Ground-floor access, including stairs and landing, is limited to 150 square feet, and the ADU can't have any inside connection to the garage below. Encinitas, California, says that if an ADU is built above a garage, the garage itself can't also be converted to an ADU. [4][6]

A garage apartment is an ADU

Here is how state ADU laws define an ADU. California's law mentions garages directly; the others don't say what can sit below the unit:

• California: an ADU is "an attached or a detached residential dwelling unit that provides complete independent living facilities for one or more persons" on a lot with a main home, with permanent provisions for "living, sleeping, eating, cooking, and sanitation." State law lets ADUs be attached to or inside the main home, "including attached garages," or detached on the same lot, "including detached garages." The state housing department (HCD) lists "An ADU built on top of an attached garage" as an example of an attached ADU.
• Washington: a "detached accessory dwelling unit" is one "that consists partly or entirely of a building that is separate and detached from" the main home.
• Arizona: an ADU is "a self-contained living unit" on the same lot as a larger single-family home, with "its own sleeping and sanitation facilities."
• Montana: an ADU is "a self-contained living unit on the same parcel as a single-family dwelling of greater square footage that includes its own cooking, sleeping, and sanitation facilities."

See what an ADU is for which states have statewide ADU laws. [1][2][7][8][9][10]

Height: the rule that decides a second story

A unit above a garage needs room for two levels, so the height limit matters most. State laws set the lowest limit a city may impose:

• California: a city can't set a height limit lower than 16 feet for a detached ADU; 18 feet within a half-mile walk of a major transit stop or high-quality transit corridor, plus 2 more feet to match the main home's roof pitch; and 18 feet on a lot with a multifamily, multistory building. For an ADU attached to the main home, the minimum is 25 feet or the zoning height limit for the main home, whichever is lower. The law "shall not require a local agency to allow an accessory dwelling unit to exceed two stories." HCD adds that if a detached two-story ADU fits within the required heights and meets the building code, a city "cannot deny an ADU application to create a two-story ADU," even where zoning limits the main home to one story. HCD's example of an attached ADU is one "built on top of an attached garage"; ask your city which height limit it applies to a unit over a detached garage.
• Washington: covered cities and counties "may not establish roof height limits on an accessory dwelling unit of less than 24 feet," unless the main home's limit is lower, in which case the ADU limit can't be lower than the main home's.
• Arizona: cities of more than 75,000 people can't set rules for ADUs that are "more restrictive than those for single-family dwellings within the same zoning area with regard to height."
• Montana: cities and towns that zone can't set maximum building heights for ADUs "that are more restrictive than those for the single-family dwelling on the lot."

These are floors. Cities can allow more, and some set a separate limit for a unit over a garage. Check your city's ADU rules, and see your state page for the rest of your state's law. [3][7][9][10][11]

Setbacks when you build on top of an existing garage

In California, the setback rule depends on whether you are reusing an existing structure. State law says no setback may be required for "an existing living area or accessory structure or a structure constructed in the same location and to the same dimensions as an existing structure that is converted to an accessory dwelling unit." For any other ADU, the side and rear setback can be no more than four feet.

A second story is new space, so ask your city how it treats it. Two California cities answer in their own rules:

• Burbank: the side and rear setback is 4 feet, "including ADUs constructed on top of a garage or accessory structure." When a legally permitted garage is converted, its existing nonconforming setback can stay, but the 4-foot setbacks "still apply to all added space that goes beyond the existing building footprint and/or building envelope of the garage."
• Encinitas: "A minimum setback of four feet shall be required for an ADU constructed above an existing garage," while setbacks don't apply to an ADU converted from an existing permitted structure.

In Washington, covered cities must allow ADUs converted from existing structures, "including but not limited to detached garages, even if they violate current code requirements for setbacks or lot coverage." Ask your city whether an addition on top counts as a conversion. [2][4][6][11]

Parking: keeping or replacing the garage spaces

If you tear down the old garage to build a new two-story one, or convert it, California law says that when a garage, carport, covered parking structure or uncovered parking space "is demolished in conjunction with the construction of an accessory dwelling unit or converted to an accessory dwelling unit, the local agency shall not require that those offstreet parking spaces be replaced." Burbank's ADU handout says likewise: when an existing garage, carport or covered parking structure is "demolished or converted to facilitate the construction of an ADU, that parking does not have to be replaced."

City codes don't always match. Encinitas's code says an ADU "may be constructed above a garage provided that there is no loss of parking provided within the garage," and that required parking removed to build an ADU "shall be replaced on the same lot." The city's May 2025 ADU guide lists the replacement rule only for properties in the Coastal Zone, west of Coast Highway 101, between Swami's and La Costa Avenue. HCD says state ADU law applies in the Coastal Zone but does not "alter or lessen the effect or application of Coastal Act resource protection policies." Confirm with the Encinitas Development Services Department at (760) 633-2710 which rule applies to your lot.

In other states, parking limits are about the ADU itself: Arizona and Montana cities may not require that a lot have additional parking for an ADU, and covered Washington cities can't require off-street parking for an ADU within half a mile walking distance of a major transit stop. [2][6][7][9][10][11][12][13]

Fire separation and other building code points: one city's example

Your city's adopted building code sets the rules for the floor, walls and exit between a garage and a home above it. Minneapolis's building code handout for detached ADUs, which it says includes "an addition above a detached garage" (last updated December 10, 2024, under the 2020 Minnesota Residential Code), lists these points:

• The garage must be "separated from dwelling unit, including the exit, with one-hour fire assembly and 20-minute doors."
• "The ADU exit shall discharge directly outside the structure without entering the garage, or through a fire-rated stair."
• The ceiling between the garage and the unit needs R-30 insulation.
• The unit can't share a ventilation system with the garage and needs its own heating and ventilation.
• "Buildings containing dwellings must have frost footings. Existing detached garages are not typically on frost-protected footings." Ask your building department what an existing garage's footings need before you build on top.

These are Minneapolis's requirements; ask your own building department what its code requires. On sprinklers, California says they "shall not be required in ADUs or JADUs if sprinklers are not required by building standards for the existing primary residence." Arizona cities covered by its ADU law may not require an ADU "to comply with a commercial building code or contain a fire sprinkler." [5][7][9]

City examples

• Burbank, CA: an ADU built on top of a detached garage can be 20 feet to the top plate and 23 feet to the top of the roof, compared with 17 feet for a new detached ADU that isn't on top of a garage or other accessory structure (18 feet near transit).
• Portland, ME: "Detached ADUs shall be limited to a maximum height of 18 feet, unless constructed above a garage, in which case the height of the structure shall be limited to 25 feet."
• Encinitas, CA: 4-foot setback above an existing garage; no loss of garage parking under the code; the garage below can't also become an ADU.
• Minneapolis, MN: one-hour fire separation between the garage and the unit, and an exit that goes directly outside without entering the garage, or through a fire-rated stair.

For costs, see our ADU cost guide. For the permit steps, see the ADU permit process. [4][5][6][13][14]

Questions people ask