Guides / ADUs · Updated
What California lets you build as an ADU
From the state housing department's ADU Handbook (March 2026), with the Government Code sections it cites, checked November 2026.
Short answer
Every California city has to let you build an 800 sq ft ADU with 4 ft side and rear setbacks, up to 16 ft tall (18 ft near major transit). On a single-family lot you can have a detached ADU, a converted one and a junior ADU of up to 500 sq ft. Under 750 sq ft there are no impact fees, and near transit there's no parking requirement.
The rules, one by one
| Rule | What the law says | Gov. Code |
|---|---|---|
| Size every city must allow | At least an 800 sq ft ADU with 4 ft side and rear setbacks, whatever the local lot coverage, floor area or open space rules | § 66321(b)(3) |
| Largest a city may cap it at | Its maximum must allow at least 850 sq ft, or 1,000 sq ft with more than one bedroom. Without a compliant local ordinance, a new detached ADU can be 1,200 sq ft | § 66321(b)(2), § 66314(d)(4) |
| Side and rear setbacks | No more than 4 ft | § 66321(b)(3) |
| Height, detached | 16 ft; 18 ft within a half-mile walk of a major transit stop or a high-quality transit corridor, plus 2 ft to match the house’s roof pitch | § 66321(b)(4) |
| Height, attached | Up to 25 ft, or the house’s limit if that’s lower; no third storey required | § 66321(b)(4)(D) |
| How many on a single-family lot | At least one ADU made from existing space, one junior ADU and one new detached ADU | § 66323(a) |
| Junior ADU | No more than 500 sq ft, inside the house (an attached garage counts as part of it) | §§ 66333–66339 |
| Parking | None near transit, in a historic district, inside the house or an existing structure, with a car share within a block, or where street permits aren’t offered; otherwise at most one space per unit or per bedroom | § 66322(a), § 66314(d)(10) |
| Impact fees | None for an ADU of 750 sq ft or less of interior living space | § 66311.5(c) |
| Owner-occupancy | Can’t be required for an ADU; a city may require rentals of 30 days or longer | § 66315 |
These are the floor every city must allow. A city's own ordinance can be more generous, never stricter on these points, so read your city's ADU page too.
In the law's words
A city can't set limits "that does not permit at least an 800 square foot accessory dwelling unit with four-foot side and rear yard setbacks to be constructed in compliance with all other local development standards" (§ 66321(b)(3)).
On single-family lots, "local agencies must allow at least one ADU constructed from existing space, one JADU, and one newly constructed detached ADU."
No standards beyond the state's "shall be used or imposed, including an owner-occupant requirement, except that a local agency may require that the property may be used for rentals of terms 30 days or longer" (§ 66315).
See it on your lot
The ADU floor plans guide has San Diego County's six free plans drawn to scale, from 600 to 1,200 sq ft. Place one on your lot with the measuring map, turn on the property lines, and set the setback to 4 ft to see how much room it leaves. Building elsewhere? See the rules in Washington and Oregon.