The fundamental difference
A detached ADU is a standalone small home in your backyard, separated from the main house. An attached ADU shares at least one wall with the main house — sometimes as an addition, sometimes carved out of existing space.
Both qualify as ADUs under California state law. Your choice changes timeline, cost, privacy, and resale value.
When detached is the right call
- You want maximum privacy for renters or family members.
- Your backyard has enough buildable area after setbacks (typically four feet on the sides and rear).
- You plan to potentially sell the unit separately under AB 1033.
- You want clear visual separation from the main house.
Tradeoffs: Typically higher build cost, longer utility runs, and more site work.
When attached is the right call
- Your lot is narrow or small, limiting backyard footprint.
- You want a faster permit timeline.
- You want shared HVAC and utility infrastructure to reduce cost.
- The ADU will be occupied by family who don't need full visual separation.
Tradeoffs: Less privacy, harder to sell separately, and shared roof/wall construction details add complexity.
What does not change between the two
- State law guarantees both up to 1,200 sq ft.
- Both can have full kitchens and bathrooms.
- Both qualify for the same financing options.
- Both increase property value when done correctly.
Garage conversion: a third option
A garage conversion is technically an attached ADU when the garage is attached to the main house, or detached when it is a standalone structure. Garage conversions usually have the lowest cost and fastest permitting because the footprint already exists.
How to decide on your specific lot
Walk your yard with a tape measure. Measure your buildable area after subtracting setbacks. If you have at least 30 feet by 25 feet of clear backyard, detached is viable. If less, attached is usually the better path.
Related reading
Frequently asked questions
Does a detached ADU add more property value than attached? Typically yes, because it has more resale flexibility (especially under AB 1033).
Can an attached ADU be on a second story? Yes, in cities that allow two-story ADUs.
Is permitting faster for one or the other? Attached ADUs and garage conversions usually permit faster than detached new construction.