ADU builders
Detached ADU construction: what the build takes
By the HyreADU research desk. Last checked .
The short answer
Detached ADU construction means building a new, separate unit, usually in the rear yard.
It needs everything a house needs: its own foundation, framing, roof, and utility runs from the street or main house.
In California a detached unit of up to 800 square feet with four-foot side and rear setbacks must be approved if it meets the code.
What the build includes
Site prep and the foundation; framing and the roof; rough plumbing, electrical and mechanical; insulation and drywall; finishes; then final inspection. Utility trenching to the unit often runs in parallel. See ADU construction for the phases.
The California floor
Gov. Code § 66323(a) requires ministerial approval of one detached new-construction ADU with four-foot side and rear setbacks on a lot with a house.
Section 66321 sets a height floor of 16 feet, or 18 feet near transit with two more for roof pitch.
A newly built detached unit is also a new building under the Energy Code, so solar rules can apply.
Who builds it
A contractor with a California B General Building license. Ask how many detached ADUs they have finished in your city, who handles the utility applications, and whether site work is a priced line.
Not sure detached is right? Compare it with an attached ADU or use the ADU type selector.
What to do next: read the questions to ask an ADU builder, check what decides ADU cost, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
How big can a detached ADU be?
Local rules set the maximum. California bars a maximum below 850 square feet, or 1,000 for more than one bedroom (Gov. Code § 66321).
Does a detached ADU need its own address and meter?
Address rules are local. On meters, see ADU utility connection.
Can I build a detached ADU and a JADU?
California allows certain combinations on a single-family lot under Gov. Code § 66323. Check your city.
How long does detached construction take?
Use the ADU timeline planner with your builder’s numbers.
Sources
- Gov. Code § 66323 (by-right ADUs, utility connections). Checked September 5, 2026.
- Gov. Code § 66321 (size and height floors). Checked September 5, 2026.
- CSLB Fast Facts: Accessory Dwelling Units (2025). Checked October 27, 2026.
- California Energy Commission, 2022 Energy Code update summary. Checked September 5, 2026.
HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax or engineering advice. Rules vary by state and city; California rules are labeled as such.