How big and how tall can your ADU be?

Most ADU designs start with two questions: how many square feet can I build, and how tall can it be? The answer comes from your city's ordinance, but in some states the city cannot go below a state floor.

California: the floors cities must allow

California's ADU handbook explains the state rules:

• Size. A city can set minimum and maximum sizes, but its maximum must allow at least 850 square feet of interior living space, or 1,000 square feet for an ADU with more than one bedroom.
• The 800 square foot guarantee. Lot coverage, floor area ratio, open space and front setback rules cannot stop an ADU of at least 800 square feet with four-foot side and rear setbacks.
• No local ordinance? If a city has no compliant ADU ordinance, the maximum for a new detached ADU is 1,200 square feet. An attached ADU can be up to 50 percent of the main house's floor area, but at least 800 square feet must be allowed.
• Small is allowed. Cities cannot set a minimum that blocks an efficiency unit, which can be as small as 150 square feet.
• Conversions are different. Converting an existing structure or part of the house is not subject to size limits.
• Height. The base height limit for a detached ADU is 16 feet. It rises to 18 feet within a half-mile walk of a major transit stop or high-quality transit corridor, and up to 20 feet in some conditions.
• Bigger is possible. Cities can allow more than 1,200 square feet if they choose.

Washington: two units and 1,000 square feet

Washington's 2023 law, HB 1337, applies to cities and counties inside urban growth areas. According to the state Commerce Department's guidance, they must allow two ADUs per lot, in any mix of attached and detached, and must allow an ADU of at least 1,000 square feet. The Puget Sound Regional Council's permit checklist adds that ADUs must be allowed to be at least 24 feet tall, unless the main house is held to a lower height, and cannot face stricter setback, design review or lot coverage rules than the main house.

Other states

Oregon, Texas and Florida leave size and height largely to cities. Austin, for example, says the allowed size of an ADU depends on the zoning district and location in the city, and a lot must be at least 5,750 square feet to qualify under its ADU rules. Florida's statute simply allows local governments to adopt an ordinance to allow ADUs and leaves the details to them. See our state rules guide.

Design rules to check locally

• Setbacks from side, rear and front lot lines
• Distance between the ADU and the main house. Austin, for one, removed its zoning separation rule but still requires building code fire separation.
• Entry door location and window placement facing neighbors
• Roof pitch, materials or style matching the main house
• Parking, which several states now limit
• Tree protection

Why size is also a money decision

Size drives cost, fees and rent. In California, staying at or under 750 square feet avoids impact fees, and under 500 square feet avoids school fees. The San Diego pilot found the per-foot cost falls as units get bigger, but the total cost climbs. Its 990 square foot two-bedroom cost $296,832, more than double the one-bedroom.

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