What you can build

How big can an ADU be? Size limits by state

Checked against state laws and official agency sources · Last reviewed November 5, 2026

How big can an ADU be in California?

California's housing agency (HCD) sums up the rules in its ADU Handbook (March 2026, reflecting the law as of January 1, 2026):

• The size cities must allow: a city "may, by ordinance, establish minimum and maximum unit size requirements for both attached and detached ADUs," but the maximum "must allow an ADU of at least 850 square feet of interior livable space, or 1,000 square feet of interior livable space for ADUs with more than one bedroom."
• The 800-square-foot ADU: lot coverage, floor area ratio and open space rules can't stop an ADU "of at least 800 square feet of interior livable space that maintains four-foot rear and side setbacks."
• Without a compliant local ordinance: the maximum is "1,200 square feet of interior livable space for a new detached ADU" and "up to 50 percent of the floor area of the existing primary dwelling for an attached ADU (at least 800 square feet of interior livable space)."
• Attached ADUs: a city may cap an attached ADU at a percentage of the house (for example, 50 percent), "but only if it does not restrict an ADU's size to less than 850 square feet of interior livable space, or 1,000 square feet of interior livable space for ADUs with more than one bedroom."
• Bigger is allowed: "Maximum unit sizes can exceed 1,200 square feet for ADUs through the adoption of a less stringent local ADU ordinance."
• Conversions: turning an existing accessory structure, such as a garage, or part of the house into an ADU "is not subject to unit size requirements."
• Junior ADUs: no more than 500 square feet of interior livable space, inside a single-family home.
• Smallest: a city can't set a minimum size that rules out an efficiency unit, which "may be as small as 150 square feet in floor area."

Height in California. A city can't set a height limit below 16 feet for a detached ADU. It must allow 18 feet within a half mile of a major transit stop or high-quality transit corridor, plus 2 more feet for a roof pitch that matches the house, and 18 feet on a lot with a multistory multifamily building. An attached ADU can be 25 feet or the height limit for the house, whichever is lower. Where the height limit allows, the city must allow at least two stories. See our two-story ADU guide.

Your city's own ordinance may allow more. See our California page and the California city pages linked from it. [1]

ADU size limits by state

What state law says about ADU size in each state we cover. Where the state sets no size rule, your city or county does. Most of these laws cover only some cities or counties, and dates and details are on each state page.

[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]

What counts toward an ADU's size

Each law and code measures size its own way, so check which measure yours uses:

• California measures interior livable space: "a space in a dwelling intended for human habitation, including living, sleeping, eating, cooking, or sanitation." HCD adds that it includes "bedrooms, bathrooms, kitchens, laundry rooms, closets, and interior stairs."
• Washington uses gross floor area, Connecticut net floor area, Vermont the home's "total habitable floor area," and New Hampshire "total living space."
• Seattle leaves out of its ADU size limit "Up to 250 square feet of gross floor area in an attached garage," "All stories, or portions of stories, that are underground," and up to 35 square feet for long-term bicycle parking.
• Portland, Oregon measures living area, after the ADU is built. Its size limit "does not apply when the basement of a primary dwelling unit is converted to an accessory dwelling unit and the primary dwelling unit has been on the site for at least 5 years."
• Conversions in California: converting existing space in the house or an existing accessory structure "is not subject to unit size requirements." See our garage conversion guide and basement apartment guide.

Ask your city how it measures, and whether garages, basements, porches or lofts count, before you draw plans. [1][2][6][17][23][26][27]

How tall can an ADU be?

Height is usually set by the city, but several state laws put a floor under it:

• California: at least 16 feet for a detached ADU, 18 feet near major transit (plus 2 feet for a matching roof pitch), and 25 feet or the house's limit, whichever is lower, for an attached ADU.
• Washington: cities and counties "may not establish roof height limits on an accessory dwelling unit of less than 24 feet," unless the house's own limit is lower.
• Idaho: cities over 10,000 can't set a height limit "less than the height of an existing single-family primary dwelling on a lot."
• Arizona, Arkansas, Iowa, Montana and West Virginia (and Virginia from July 1, 2027): height limits for an ADU can't be stricter than for the house or for single-family homes in the same zone.

For building up, see our two-story ADU guide. [1][2][3][4][10][11][15][24][25]

Junior ADUs and ADUs inside the house

A junior ADU is a California unit "no more than 500 square feet of interior livable space in size and contained entirely within a single-family residence," including an attached garage. See our junior ADU guide.

Elsewhere, ADUs inside the house have their own rules. In Utah, a city can't regulate "the size of the internal accessory dwelling unit in relation to the primary dwelling." In Washington, D.C., an accessory apartment "may not occupy more than thirty-five percent (35%) of the gross floor area of the house," and the house must have at least 2,000 square feet (R-1 zones) or 1,200 square feet (R-2 and R-3 zones), not counting garage space. [1][7][21]

City examples

State rules set the floor; cities set the actual numbers. Some examples from our city pages:

• Seattle: "The gross floor area of an accessory dwelling unit with up to two bedrooms may not exceed 1,000 square feet," and 1,200 square feet with three or more bedrooms.
• Portland, Oregon: "no more than 75 percent of the living area of the primary dwelling unit or 800 square feet of living area, whichever is less."
• Phoenix: on lots up to 10,000 square feet, 1,000 square feet or 75% of the house's gross floor area, whichever is less. On larger lots, 3,000 square feet, 10% of the net lot area or 75% of the house, whichever is less.
• Denver, single-unit zone districts: 864 square feet on zone lots of 7,000 square feet or less, and 1,000 square feet on larger lots.

For your city, start with its page in our list of states and cities, or ask the city's planning department. [26][27][28][29]

Questions people ask