ADU types
Detached ADU: the rules, the size and where it can go
By the HyreADU research desk. Last checked .
The short answer
A detached ADU is a separate small home on the same lot as a main house.
In California, a city must approve one detached new-construction ADU with four-foot side and rear setbacks on a lot with a single-family home (Gov. Code § 66323(a)), must allow at least 16 feet of height (18 near transit), and cannot set a maximum size below 850 square feet, or 1,000 with more than one bedroom.
The California rules that matter
- By-right approval of one detached ADU with four-foot side and rear setbacks on a single-family lot (Gov. Code § 66323(a)).
- Height of at least 16 feet; 18 feet near transit plus 2 feet for roof pitch (§ 66321(b)(4)).
- A maximum size no lower than 850 sq ft, or 1,000 sq ft for more than one bedroom (§ 66321(b)(2)).
- Room for at least 800 sq ft with four-foot setbacks may not be blocked by other standards (§ 66321(b)(3)).
- No owner-occupancy requirement (§ 66315).
- A 60-day decision on a complete application (§ 66317).
Where it can go on your lot
Where a detached ADU can go depends on the open area after setbacks, easements, trees and the main house. It also depends on how far the sewer and power must travel. The ADU setback calculator shows the pad left after setbacks, and the size envelope calculator adds coverage and the local maximum you type.
What it takes to build
- A new foundation suited to your soil.
- Utility runs: water, sewer or septic, and power, sometimes gas.
- A roof, walls and windows built to the energy code.
- Access for materials, or for a crane if it is prefab.
Builders who do this work are covered on detached ADU construction.
Outside California
Rules vary by state and city. Seattle calls detached units DADUs; see ADU vs DADU. Several states have passed ADU laws in recent years; our laws by state research covers those we have read.
A mistake to avoid
The mistake to avoid is placing the ADU before finding the sewer line. A unit at the far back corner may need a long trench, or a pump if the ground slopes away from the street. Ask a plumber where the lateral runs before you place the building.
What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
How close to the property line can a detached ADU be?
In California, a city cannot require more than four feet from side and rear lines for the by-right detached ADU.
Can a detached ADU be two stories?
Only if the local height limit allows; the state floor is 16 feet.
Do I need parking for a detached ADU?
At most one space in California, and often none under state exemptions.
Is a detached ADU the same as a DADU?
Yes; DADU is the Seattle term.
Sources
- Gov. Code § 66323 (ADUs a city must approve ministerially). Checked September 5, 2026.
- Gov. Code § 66321 (size and height standards a city may not impose). Checked September 5, 2026.
- Gov. Code § 66315 (no extra standards; rentals of 30 days or longer). Checked September 5, 2026.
- Gov. Code § 66317 (ministerial review, completeness and the 60-day decision). Checked September 5, 2026.
- Seattle SDCI, Accessory Dwelling Unit (backyard cottage). Checked October 27, 2026.
HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax, lending or engineering advice.
Rules vary by state and city; California rules are labeled as such. Spotted an error? Tell us through our corrections page. Licensed professionals can also contribute as named sources.