What you can build

Can you build a two-story ADU?

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

The short answer

Whether you can build a second story depends on your city's height limit for ADUs, and on whether your state makes the city allow a certain height.

• States with a height floor in feet: California and Washington set minimum heights a city must allow.
• States with a "no stricter than the house" rule: Arizona, Montana and Massachusetts bar ADU height rules that are stricter than those for a house. Idaho's 2026 law tells larger cities to stop capping ADU height below the existing house's height.
• States that leave height to cities: Oregon's law makes cities allow ADUs but leaves siting and design to "reasonable local regulations" and sets no height.

Then check your city's code. Many cities allow more than the state minimum, and some set separate limits for one-story and two-story ADUs. [1][2][3][4][5][8][9]

California: the heights every city must allow

Under Gov. Code 66321(b)(4), a city or county can't adopt a height limit that doesn't allow at least:

• 16 feet for a detached ADU on a lot with an existing or proposed single-family or multifamily home.
• 18 feet for a detached ADU within a half-mile walk of a major transit stop or high-quality transit corridor, plus 2 more feet for a roof pitch that matches the main home's.
• 18 feet for a detached ADU on a lot with an existing or proposed multifamily, multistory building.
• 25 feet, or the zone's height limit for the main home if that is lower, for an ADU attached to the home. This rule doesn't require a city to allow an attached ADU above two stories.

These are minimums. HCD, the state housing department, says "There is no limit on the number of stories contained in State ADU Law." It says a city "must allow at least two stories, if this can be accommodated under the applicable height limit while complying with all applicable building standards."

For detached ADUs, HCD says that if a two-story ADU fits within the state height allowances and meets the building code, a city "cannot deny an ADU application to create a two-story ADU, irrespective of the underlying zoning that might restrict a primary dwelling to one story."

A conversion is different. HCD says an ADU converted from existing space is limited to the height of the original structure.

See California ADU rules for the rest of the state law. [1][10]

Washington, Oregon and other states

• Washington: Under RCW 36.70A.681, a city or county covered by the law "may not establish roof height limits on an accessory dwelling unit of less than 24 feet." If the main house has a height limit under 24 feet, the ADU limit can't be lower than the house's.
• Oregon: ORS 197A.425 makes cities over 2,500 people and counties over 15,000 allow at least one ADU per detached house inside urban growth boundaries, "subject to reasonable local regulations relating to siting and design." It sets no height, so your city decides.
• Montana: Under MCA 76-2-345, a city or town may not set maximum building heights for ADUs "that are more restrictive than those for the single-family dwelling on the lot."
• Arizona: ARS 9-461.18 applies to cities and towns of more than 75,000 people. They may not set ADU height rules "more restrictive than those for single-family dwellings within the same zoning area."
• Massachusetts: Under 760 CMR 71.03, a town can't hold a protected-use ADU to height or number-of-stories rules stricter than those for the main home, a single-family home or an accessory building in the district, whichever is more permissive.
• Idaho: Idaho Code 67-6541 applies to cities of more than 10,000 people. By February 1, 2027, they must change their rules to bar a building height limit for an ADU that is "less than the height of an existing single-family primary dwelling on a lot."

In these states the ADU's height limit is tied to the limit for the house (in Idaho, to the existing house's height). Your city's zoning sets the actual number. [2][3][4][5][8][9]

City examples: stories and heights

Cities can allow more than the state minimum. Some examples from our city guides (some name a number of stories, others only a height):

• San Diego: ADUs follow the overall height limit of the base zone and any overlay zone. On lots that allow single-family but not multifamily homes, a detached ADU "shall not exceed two stories."
• San José: A detached one-story ADU can be 18 feet tall and a detached two-story ADU 25 feet. A detached ADU can't exceed two stories.
• Torrance: Citywide, a detached ADU can be 18 feet for one story or 23 feet for two stories. In the Hillside Overlay and Coastal Zone, a detached ADU is 16 feet for one story, or more where Gov. Code 66321 allows it.
• Denver: In the Urban (U-) neighborhood zone districts, the detached ADU building form allows 2 stories and 24 feet, within a bulk plane measured from the side lot lines.
• Mesa: The city answers "Yes" to building a two-story detached ADU behind a one-story house, because ADU height follows the limit for single-family homes in your zone. Any garage, workshop or other uninhabitable space in a detached ADU building must meet the height rules for detached accessory buildings.
• Portland, Oregon: The city sets a height, not a number of stories: a detached ADU can be 20 feet tall outside the required setbacks and 15 feet inside them.

A conflict to watch in Tempe. Tempe's ADU page says an ADU's height is limited to the zone's limit, "typically 30'," but that "Based on the December 17, 2025 Board of Adjustment decision, the City of Tempe requires a Use Permit for two-story ADU designs." Arizona's law says covered cities must allow ADUs as a "permitted use," meaning approval without "a public hearing, variance, conditional use permit, special permit or special exception." Confirm with Tempe Planning before you design a second story. [3][6][7][11][12][13][14][15]

An ADU over a garage

Putting the ADU on top of a garage is a common way to get a second story. Some cities give it its own height limit:

• Burbank: A detached ADU built on top of a garage, an accessory structure or another ADU is limited to 20 feet to the top of plate and 23 feet to the top of the roof. Other new detached ADUs are limited to 17 feet, or 18 feet near major transit (plus 2 feet for a roof pitch matching the house) or on a lot with a multistory apartment building. These numbers come from Burbank's 2023 ordinance and July 2024 handout. In a December 2025 letter, HCD told the city that the latest Burbank ADU ordinance on file with HCD was from 2020 and "may be outdated," so confirm current limits with Burbank Planning.
• Portland, Maine: Detached ADUs are limited to 18 feet, "unless constructed above a garage, in which case the height of the structure shall be limited to 25 feet."
• San Diego: An ADU attached to an existing accessory structure, such as a garage, is held to two stories on lots that allow single-family but not multifamily homes.

If you want to turn the garage itself into living space instead, see our garage conversion guide. In California, an ADU converted from an existing structure is limited to that structure's height. [10][11][16][17][18]

Stairs, setbacks, windows and balconies

A second story brings rules about where the stairs go and what neighbors can see. These differ a lot by city. Examples:

• Second-story setbacks: San José requires at least 4 feet from the side and rear lot lines for any second story of a detached ADU. San Diego requires an ADU taller than 16 feet next to a residential lot to keep a 4-foot side and rear setback (or the zone's setback, if less).
• Stairs and decks: In Burbank, a staircase or landing for a second-story ADU must be at least 4 feet from the rear and side lot lines. In San Diego, "Exterior staircases, decks, and balconies must conform to the base zone setback." Torrance requires new stairways to be "completely enclosed and part of the interior of the structure," except for converting an existing stairway.
• Balconies: Torrance's code says "New balconies, full height windows or doors openings with a guardrail, roof decks, and decks greater than 2 feet above grade are prohibited."
• Windows and privacy: For ADUs under its local standards, Palo Alto says second-story doors and decks "shall not face a neighboring property line." Second-story decks and balconies need solid screens at least 5 feet 6 inches tall. Second-floor windows facing a side or rear neighbor, other than egress windows, need a 5-foot sill or opaque glass.

California cities and state law. Torrance's code says that where it conflicts with state law, "State law shall govern." Palo Alto applies its privacy rules only to ADUs that don't qualify under its section for units exempt from local rules, which covers, for example, a new detached ADU of up to 800 square feet within the state heights. If a city rule seems to block an ADU that state law protects, ask the city which rules apply to your project. [6][7][11][16][19]

Building code and cost

Zoning height limits aren't the only limit. San Diego's bulletin says ADUs must meet the California Building Standards Code, "including requirements for fire separation distance, opening protection, allowable height, and allowable area," and that these "may result in increased setback requirements." HCD's two-story rule also depends on the ADU "remaining compliant with the building code." Ask your building department about these early, before the design is final.

For what a two-story ADU might cost to build, and the fees you'll pay, see our ADU cost guide. For the steps from zoning check to final inspection, see the ADU permit process. [10][11]

Questions people ask