Detached ADU or garage conversion: how to choose
Backyard cottage or garage conversion? Compare space, timeline, permits, privacy and resale value to pick the ADU that fits your lot and your budget.
The two most common ways to add an ADU are to build a new detached unit in the backyard or to convert an existing garage. Both create a separate home on your lot, but they differ on almost everything else.
Garage conversion
Why people choose it
- The structure, roof and slab already exist, so there is less new construction.
- It usually takes less time than a new building.
- It fits lots with no room for a new structure.
What to check
- The slab may need a moisture barrier and insulation, and the floor often has to be raised.
- The garage door opening is replaced by a wall with windows and a door.
- Ceiling height, insulation, ventilation and egress windows must meet the code for a dwelling.
- You lose the garage: check whether your city requires the parking to be replaced.
- An old garage with a weak foundation or framing can cost more to convert than expected. Have it inspected first.
Detached ADU
Why people choose it
- You design the unit you want: layout, size within the local limit, light and privacy.
- It is a separate building, often more private for tenants or family.
- New construction meets current energy and safety codes from day one.
What to check
- It needs a foundation, new utility lines and often trenching across the yard.
- Setbacks and maximum size limit where it can go.
- Timeline and budget are usually higher than a conversion. Prefabricated or modular units can shorten the time on site, but still need a foundation, utilities and permits.
Questions that decide it
- Who will live there? A parent who needs single-level access, a renter, an adult child?
- How much yard can you give up?
- How soon do you need the unit?
- What does your city allow for each type (size, height, parking)?
- Rental and resale: a detached unit gives a tenant more privacy; a conversion usually asks for a smaller budget up front. A local real estate agent can tell you what buyers and renters look for in your area.
Not sure? Many ADU builders offer a site visit to tell you what fits your lot. Describe your project in the form and compare their proposals.