ADU builders
Attached ADU: the rules and what to ask a builder
By the HyreADU research desk. Last checked .
The short answer
An attached ADU shares at least one wall with the main house and has its own entrance.
In California the height floor is 25 feet, or the limit for the main house if lower (Gov. Code § 66321(b)(4)(D)).
Utility runs are shorter than for a detached unit, but tying into the existing house adds its own work.
What makes it different
An attached unit joins the house structure, roof and sometimes its systems. That means a fire-rated wall or floor between the units, sound separation, and roof and flashing work where new meets old.
The Energy Code treats a new attached unit as an addition, not a new building, so the solar mandate for new buildings does not reach it.
What a builder must price
Opening up the existing wall, matching the roof, the fire separation, and any upgrade to the house’s electrical service or plumbing the new unit needs. Ask whether the bid includes repair of finishes inside the main house.
Attached vs detached
Attached gives shorter utility runs and a taller height floor. Detached gives privacy and separation. The ADU comparisons page sets the trade out. Whichever you choose, the builder needs a California B license.
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
Is an attached ADU the same as a JADU?
No. A JADU is created within the existing house, up to 500 square feet. See junior ADU.
Does an attached ADU need its own entrance?
Yes, an ADU needs independent access.
Does an attached ADU need solar?
Under the CEC’s classification it is an addition, so the new-building solar rule does not apply.
Can an attached ADU have a second story?
The 25-foot height floor allows it in California, subject to local rules.
Sources
- Gov. Code § 66321 (size and height floors). Checked September 5, 2026.
- California Energy Commission, 2022 Energy Code update summary. Checked September 5, 2026.
- CSLB Fast Facts: Accessory Dwelling Units (2025). 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 or engineering advice. Rules vary by state and city; California rules are labeled as such.