The core difference
A detached ADU is a freestanding structure built elsewhere on your lot — sometimes called a backyard home or guest house. An attached ADU shares at least one wall (and often a foundation or roofline) with your existing home. Both can be fully independent living units with their own kitchen, bathroom, and entrance — the difference is construction approach, not livability.
Side-by-side comparison
| Factor | Detached ADU | Attached ADU |
|---|---|---|
| Typical Cost | $170,000 – $420,000+ | $140,000 – $280,000 |
| Typical Timeline | 8–15 months | 6–11 months |
| Construction Approach | New foundation, framing, roof, and utilities | Shares a wall (often foundation/roofline) with the main house |
| Privacy from Main House | Most separation — fully freestanding | Less separation, but usually a separate entrance |
| Design Flexibility | Most control over layout, size, finishes | Constrained somewhat by the existing home's structure |
| Lot Impact | Uses backyard or side-yard space | Can sometimes use less total new footprint |
Figures reflect typical ranges across our Los Angeles, Orange, San Bernardino and Riverside county service area — see our full ADU cost guide for how these are calculated and what can push a specific project outside these ranges.
When a detached ADU tends to make more sense
- You want maximum privacy between the ADU and the main house — useful for pure rental units or long-term family living where everyone wants their own space.
- Your lot has usable backyard space that meets setback requirements once you check with your local zoning rules.
- You're not in a rush. New construction generally takes longer than working with an existing structure.
- Rental appeal or resale flexibility is a priority — a fully separate unit often reads as more attractive to a prospective tenant, though actual rental and resale outcomes depend on your specific property and market.
When an attached ADU tends to make more sense
- Your backyard is limited but there's room to add onto the existing structure instead.
- You want a somewhat faster, lower-cost path than a fully detached unit, since it can reuse part of an existing foundation or wall.
- Multigenerational living is the goal — many households building for aging parents or adult children specifically want some proximity to the main house while keeping a separate entrance and living space.
- You'd rather not lose backyard space to a standalone structure.
What actually decides it, in practice
For most homeowners, the decision comes down to three things: how much usable lot space you actually have once setbacks are applied, your budget and timeline tolerance, and whether privacy/separation or proximity is the higher priority. A free property review looks at your specific lot and goals rather than asking you to guess.
Frequently asked questions
Generally, yes, since it can reuse part of an existing foundation or wall, reducing sitework compared to entirely new construction. But actual costs depend on your specific design, site conditions, and finish level — see our ADU cost guide for the full picture.
In some cases, yes. An attached ADU can be new square footage built onto your home, or in some cases an interior conversion sharing a wall with the rest of the house. Which approach fits depends on your existing floor plan.
Many households choose an attached ADU for multigenerational living specifically because it keeps family close while still providing a separate entrance and independent living space. That said, some households prefer the fuller separation a detached unit offers — it depends on the specific dynamic you're planning for.