An ADU starts with the lot, not the floor plan. A few hours of homework tells you whether the idea is worth a builder's visit and makes that first conversation far more useful.
Know your parcel and lot lines
- Pull your parcel map from the county assessor or GIS site.
- Find your survey or plat if you have one. Fences are not always on the line.
- Note where the house, garage and sheds sit relative to each line.
Zoning, setbacks, lot coverage and height
Your city or county sets how close you can build to each line, how much of the lot can be covered and how tall the ADU can be. State law shapes those rules in some places, but the local code is what gets applied to your permit. Read your state's guidance, then confirm with your planning department:
- California: HCD Accessory Dwelling Units
- Washington: Commerce ADU page
- Oregon: DLCD ADU guidance
Utilities
- Water: where the meter and main line are, and whether a separate meter is required.
- Sewer or septic: where the sewer tap is and how deep. On septic, whether the system can take another unit.
- Electric: your panel size and whether the service can carry a second home.
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.
Access, slope, trees and easements
- Can a crew, concrete truck and materials reach the spot?
- Slope drives excavation, retaining walls and foundation cost.
- Large trees and their roots may limit where you can dig.
- Easements on your title (utility, drainage, access) are places you usually can't build.
Rough budget categories
| Category | What it covers |
|---|---|
| Design and permits | Plans, engineering, plan review |
| Fees | Impact, tap and connection fees |
| Site work | Excavation, grading, utility trenching |
| Building | Foundation through finishes |
| Contingency | The surprises every site has |
What to bring to a builder
- Address and parcel map
- Any survey, plat or title report
- Photos of the yard, panel, meter and cleanouts
- What you want the ADU for and a budget range
If you want a head start, you can type your address into the free lot check to see your lot lines and the rules on record. Treat it as a starting point and confirm with your city.