ADU comparisons
Attached vs detached ADU: height, privacy and rules
By the HyreADU research desk. Last checked .
The short answer
Attached vs detached ADU turns on privacy, height and space. A detached ADU stands apart, with a state height floor of 16 feet in California (18 near transit).
An attached ADU shares a wall with the house, can reach 25 feet or the house limit, whichever is lower, and often reuses the house’s utilities, but it needs a rated wall between the two homes.
| Factor | Attached ADU | Detached ADU |
|---|---|---|
| State height floor in California | 25 ft or the main house limit, whichever is lower (§ 66321(b)(4)(D)) | 16 ft; 18 ft near transit (§ 66321(b)(4)) |
| Size cap a city may not go below | 850 sq ft, or 1,000 with more than one bedroom (§ 66321(b)(2)) | Same |
| Privacy | Shared wall; sound and smell travel | Separate building |
| Utility runs | Usually short | Often a longer trench to the rear yard |
| Fire separation | Rated wall or floor between units under the building code | Separation by distance |
| Yard space lost | Less | More |
When an attached ADU wins
An attached ADU wins on a lot with little rear yard, or where the house already has a side that can be extended. Utility runs are short, and the taller height floor can allow a second story over an existing structure. It suits a household that wants a parent close by.
When a detached ADU wins
A detached ADU wins on privacy. Tenants and family both value a separate door, separate walls and some yard between the homes.
It is also the type that California approves by right with four-foot side and rear setbacks under Gov. Code § 66323(a), and the type most preapproved plan programs cover. Read more on the detached ADU page.
What decides it on your lot
Where the house sits on the lot usually decides this. Measure the rear yard after setbacks, find the sewer cleanout and the electrical panel, and check whether a tree or easement blocks the yard.
The ADU type selector walks through these questions, and the ADU setback calculator shows the rear-yard pad. Builders who do each type are covered on attached ADU and detached ADU construction.
A mistake to avoid
The mistake to avoid is underestimating the shared wall. Building codes require fire separation between an attached ADU and the house, and sound travels through walls that were never designed to separate households. Ask the designer how the wall will be built and insulated for sound.
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
Is an attached ADU cheaper than a detached one?
Often the utility work is shorter, but the shared wall needs fire separation and work on the existing house. We publish no price; compare bids on your drawings.
Can an attached ADU be two stories?
In California the attached height floor is 25 feet or the main house limit, whichever is lower, so a second story is often possible. Your city can allow more.
Which type holds more value?
There is no defensible general answer. See our research on whether ADUs add value.
Does an attached ADU need a separate entrance?
Yes. Every ADU needs its own exterior access.
Sources
- Gov. Code § 66321 (size and height standards a city may not impose). Checked September 5, 2026.
- Gov. Code § 66323 (ADUs a city must approve ministerially). Checked September 5, 2026.
- Gov. Code § 66313 (ADU and junior ADU definitions). Checked October 7, 2026.
- International Residential Code 2021 (R305 ceiling height, R310 emergency escape). 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, 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.