When a homeowner asks us whether an ADU will fit, the first thing we look at isn't the size of the yard. It's the setbacks. A big backyard can have a small buildable area, and a modest lot can fit a unit easily, depending on how far the building has to stay from each line.
What a setback is
A setback is the minimum distance between a building and something else, usually a property line. For an ADU there are typically four to think about:
- Rear setback: distance from the back property line.
- Side setbacks: distance from each side property line.
- Front setback: usually only matters for attached units or corner lots, because most detached ADUs sit behind the house.
- Building separation: distance between the ADU and the main house or other structures. Not every city has one, and fire code can add its own.
The area left after you pull every setback in from the lines and take out the existing buildings is your buildable area. That shape, not the lot size, decides which plans fit.
Where to find your setbacks
- Find your zoning district. Your city or county GIS map usually shows it when you click your parcel.
- Read the ADU section of the local code. Many cities have ADU-specific setbacks that are smaller than the ones for the main house. The general zoning table is often not the right number.
- Check for state limits. Some states cap what a city can require. In California, state law generally limits side and rear setbacks for an ADU to no more than four feet; see the HCD ADU Handbook below. Washington and Oregon have their own ADU laws that set floors for what cities must allow, but the setback numbers still come from the local code.
- Look at the plat or survey. Easements (utility, drainage, access) can block part of a yard even where the setback would allow a building.
What changes the buildable area
Corner lots
A corner lot often has two front setbacks, which can push the buildable area toward one side of the yard.
Alleys
Some codes measure the rear setback from the center of an alley or allow a smaller setback next to one. Check how your city measures it.
Height
In some codes a taller ADU or a second story needs a larger setback, or a height limit applies close to the line. A plan that fits as a single story may not fit as a two-story unit in the same spot.
Existing structures
Sheds, pools and detached garages take room, and a separation requirement around the main house can take more. Converting an existing garage sometimes keeps its existing (closer) position, depending on local rules.
Utilities and slope
These aren't setbacks, but they move buildings. A sewer line needs fall toward the main, and a steep yard changes foundation cost. Your builder will check both.
The Age-in-Place ADU Guide
Design a home around the life your parent wants to keep, build it so it holds up as needs change, and pay no more than it should cost.
A quick way to see it on your own lot
Get your parcel's lot lines from the county map, measure in from each line by the setback, and sketch the house and any other structures. What's left is roughly where an ADU could go. Then compare it to the footprint of the plans you like.
Many ADU builders now have a lot-check tool on their website that does this for you from an address. It's a starting point: the city's plan review is the final word, and a survey settles exactly where your lines are.
Questions to ask your builder
- Which setbacks did you use, and from which code section?
- Are there easements on my lot that block part of the yard?
- Does a two-story plan need a bigger setback here?
- Do I need a survey before design, or can we start from the county parcel lines?
Setbacks are construction and planning guidance, not legal advice. Rules change, so confirm with your city's planning department before you design.