The floor is statewide
Start with what no town can take away. One ADU by right on any single-family lot. Detached ADUs allowed, up to 950 sf by right. No special permits or discretionary hearings for a compliant unit. And no ADU-specific setbacks — your ADU follows the same setbacks as your house. That's the state floor, and we walk through every piece of it on our law page. Towns can be more generous than the floor. They can't dig below it.
What actually differs: your district's setbacks
Here's the twist most homeowners miss. The law says your ADU uses your single-family setbacks — but it never says what those setbacks are. Each town writes its own, district by district. Six real towns, primary residential district, numbers read straight from each ordinance:
| Town | District | Front | Side | Rear |
|---|---|---|---|---|
| Durham | RA | 30 ft | 10 ft | 20 ft |
| Portsmouth | SRB | 30 ft | 10 ft | 30 ft |
| Concord | RS | 25 ft | 15 ft | 25 ft |
| Manchester | R-1A | 25 ft | 10 ft | 30 ft |
| Madbury | GRA | 50 ft | 15 ft | 15 ft |
| Lee | A | 50 ft | 25 ft | 25 ft |
Read that spread again. A Manchester R-1A lot keeps a building 10 ft off the side line; a Madbury or Lee lot needs 50 ft off the road. Same state law, very different buildable envelopes. And these are just the primary districts — most towns have several, each with its own row of numbers. That's why the first question we answer on any project is “which district is your parcel in?”
Size caps differ too — upward
The state default for a detached ADU is 950 sf by right, and a town can't cap you below 750 sf. But towns are free to allow more, and some do. That matters if you're eyeing a bigger two-bedroom: the same floor plan can be by right in one town and over the cap next door. Check your town before you fall in love with a plan — our model lineup lists exact square footage for every unit so the comparison takes seconds.
The paperwork is local
By right doesn't mean no process — it means a predictable one. You still file a building permit, and every town runs its own desk: its own forms, its own fees, its own inspection schedule. A few towns still carry an owner-occupancy requirement, so we confirm that rule for your specific town before anything gets drawn. None of this is hard. It's just local, and it rewards knowing the town before you file.
Look your town up in a minute
You don't need to read your zoning ordinance — we already did. Our town directory publishes setbacks read directly from each town's official ordinance, citation included. From there, the visualizer draws those setbacks on your actual parcel and lets you drop a model inside them. And if you'd rather have a person do it, a free zoning review checks your lot against the state law and your town's numbers — no cost, no obligation.