Detached ADU
A detached ADU is a standalone structure, separate from the primary home, typically built in the backyard or side yard. It offers the most privacy for both the homeowner and the ADU occupant, and the most design flexibility since it is not constrained by the existing home's structure.
Tradeoffs: Generally the highest-cost option since it requires an entirely new foundation, roof, and structure, plus new utility connections run to a separate structure. Lot size and setback requirements also directly limit whether a detached unit fits, and how large it can be.
Attached ADU
An attached ADU is built as an addition connected to the primary home, sharing at least one wall. It can sometimes take advantage of existing utility infrastructure, which may reduce some site-work cost compared to a fully detached build.
Tradeoffs: Less privacy than a detached unit, since occupants are physically closer to the main household. Design is also more constrained by the existing home's structure and roofline.
Which fits your lot?
Detached tends to fit better when:
You have meaningful usable yard space, want maximum privacy between households, or are planning the ADU primarily as a long-term rental unit.
Attached tends to fit better when:
Your lot is narrow or has limited yard space, you want to minimize new foundation and utility work, or the unit is intended for a family member who wants proximity to the main house.
A third option: garage conversion
If you already have a garage, converting it can combine some of the cost advantages of an attached unit with a layout that functions more like a detached one. See our ADU vs. garage conversion guide for that comparison.
Setbacks and height limits affect both options
Whether you go detached or attached, local setback and height rules still apply, though California's statewide ADU law limits how restrictive those rules can be for qualifying projects. A detached unit generally has more flexibility in where it sits on the lot, while an attached unit is naturally constrained by the existing home's footprint. Neither path avoids the permit process covered in our ADU permit guide.
Privacy and long-term use matter as much as cost
Homeowners planning to rent the unit long-term often weight privacy heavily and lean detached. Homeowners building primarily for an aging parent who wants proximity and easy access to the main household sometimes prefer attached, even at a slightly higher shared-wall design complexity. There is no universally "better" choice — it depends on how you plan to use the space.
A licensed contractor who has evaluated your property in person is the most reliable source for which option actually fits your lot and budget — request a match to get that conversation started.
Have a question about your specific project?
Call (877) 300-2747 or request a match, and mention you have questions related to “Detached vs. Attached ADU” — we will route you to a licensed contractor who can speak to it directly.