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Detached ADU
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California Government Code section 66323 requires a city to allow one new detached ADU of up to 800 square feet on a lot with a single-family home, with side and rear setbacks of no more than four feet. The base height limit is 16 feet, and it rises to 18 or 20 feet under conditions set in section 66321.
What State Law Requires
Government Code section 66323 requires a city to allow one new detached ADU of up to 800 square feet on a lot that has or will have a single-family home, with side and rear setbacks of no more than four feet. Height is limited to 16, 18, or 20 feet depending on conditions in Government Code section 66321. Details are in the HCD ADU Handbook (March 2026).
| Situation | Detached ADU Height Limit |
|---|---|
| Base limit | 16 feet |
| Within a half mile walking distance of a major transit stop or high-quality transit corridor | 18 feet, plus up to 2 feet for roof pitch matching the main home |
| Lot with an existing or proposed multifamily, multistory building | 18 feet |
Larger Than 800 Square Feet
A city may allow more. State law requires every local ordinance to allow at least 850 square feet, or 1,000 square feet for a unit with more than one bedroom. Where a city has no compliant ordinance, the default cap on a new detached unit is 1,200 square feet of interior living space (HCD ADU Handbook (March 2026)). Check your city's ordinance for its limit.
Cost Range
The Angi ADU cost guide lists $110,000 to $285,000 for a new detached ADU. The Terner Center's Terner Center statewide ADU owner survey found a statewide median of $150,000 and regional medians from $100,000 to $177,500. Prices today may be higher than in a survey published in 2021. See ADU Cost In California.
Site Questions That Affect Price
- Utilities. A detached unit may be required to have its own utility connection. The distance from the street, the sewer line, and the electric panel all matter.
- Access. Equipment and materials need a path to the backyard. A narrow side yard raises labor cost.
- Grading and trees. Slopes need retaining work. Protected trees may need a permit to remove.
- Fire and utility easements. A survey shows what you can and cannot build on.
Parking, Fees, And Occupancy
A city cannot require parking for 66323 Units. For other ADUs, parking cannot exceed one space per unit or bedroom, with exemptions such as proximity to transit. An ADU of 750 square feet or less is not charged impact fees by local agencies. A city cannot require an owner to live on site for an ADU. See California ADU Rules.
Two Ways To Build
You can hire a design-build firm, or hire a designer and a separate general contractor. You can also look at factory-built units. See Prefab Versus Site-Built. Some cities offer ready-made plans, which may shorten design and plan review. See Permits And Timeline.
Related guides: Garage Conversion To ADU, ADU Financing, Questions To Ask An ADU Builder.
Common Questions
How tall can a detached ADU be in California?
Height is limited to 16, 18, or 20 feet depending on conditions in Government Code section 66321. The base limit is 16 feet, and a unit within a half mile walking distance of a major transit stop or high quality transit corridor may be 18 feet, plus up to 2 feet for a roof pitch that matches the main home.
What is the largest detached ADU a city can limit me to?
Every local ordinance must allow at least 850 square feet, or 1,000 square feet for a unit with more than one bedroom. Where a city has no compliant ordinance, the default cap on a new detached unit is 1,200 square feet of interior living space, per the HCD ADU Handbook.
How close to the property line can a detached ADU be?
A city cannot require side and rear setbacks of more than four feet for a new detached ADU of up to 800 square feet. A city may still apply front yard setbacks, but they cannot prevent an ADU of at least 800 square feet.
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