Detached vs. attached ADU
The first real decision in any ADU project is the type. The two most common builds — detached and attached — differ on cost, privacy, and how they touch your existing home. This guide compares them so you can shortlist before you budget.
Detached ADU
A free-standing unit in the backyard or side yard.
- Cost: highest per-square-foot — separate foundation, site prep, and utility runs to the main house.
- Privacy: best — fully separate building with its own entrance.
- Build: closest to building a small house; longest timeline.
- Permits: full building permit; local setbacks and lot-coverage limits apply.
Attached ADU
Built onto the primary home — a bump-out, above-garage, or converted space.
- Cost: lower — shares a wall/foundation and nearby utilities.
- Privacy: good if given a private entrance and sound separation.
- Build: less site work; must match existing roofline/foundation.
- Permits: building permit; same local rules, often easier utility tie-in.
Garage conversion — the budget path
A garage conversion (detached garage turned into living space, or a conversion that stays attached) is usually the lowest-cost legal ADU because the shell already exists. The spend goes into interior build-out: egress, a full bath, a kitchenette, separate electrical panel, and heating/cooling. It can be detached (separate garage) or attached (integral garage).
How to choose
| Factor | Detached | Attached |
|---|---|---|
| Build cost | Higher | Lower |
| Privacy | Best | Good (with design) |
| Construction time | Longer | Shorter |
| Best when | You have yard space & want separation | You want lower cost & fast tie-in |
Figures are general planning guidance. Actual cost depends on your local labor, finishes, and site. Model it with the ADU cost calculator.
Frequently asked questions
Which ADU type is cheaper to build?
Attached and garage-conversion ADUs are usually cheaper because they share structure, foundation, and utilities with the main home. Detached units add site work, a separate foundation, and longer utility runs. Use the ADU cost calculator to compare for your size and state.
Which ADU type is more private?
A detached ADU is the most private — a separate building with its own entrance. An attached unit shares a wall with the primary home, so sound separation and a private entrance need design attention.
Do detached and attached ADUs permit differently?
Both typically require a building permit; the authority (city or county) and local size, setback, and parking rules apply to either type. Check your state ADU rules page for the specifics that apply in your location.