One ADU, by right, on any single-family lot
“By right” is the phrase that changed everything. It means that if your ADU meets the rules, the town has to approve it. No special permit. No variance. No standing in front of a board on a Tuesday night hoping the neighbors don't show up. Before this law, many towns treated an ADU as a favor they could grant or deny. Now a compliant ADU gets reviewed like any other building permit.
The law applies to every single-family lot in the state — city, suburb, or dirt road. If you want the full breakdown of what changed and what didn't, our law page keeps it current, and you can read the statute text straight from the state if you like your reading dry.
Detached is finally on the table
Before July 2025, plenty of towns only allowed attached ADUs — an apartment over the garage or a converted wing of the house. The new law says detached ADUs are allowed, period. A separate small home in the backyard is now a legal option on every single-family lot in New Hampshire.
The by-right size cap for a detached unit is 950 sf. Two more size rules worth knowing: when a town's zoning says nothing about ADU size, there is no minimum, and a town can't limit an ADU to less than 750 sf. Some towns go the other way and allow units larger than 950 sf — towns are always free to be more generous than the state floor. Every one of our models lists its square footage, so you can see exactly which ones fit the by-right cap in your town.
Your setbacks are your house's setbacks
This is the part people ask about most. Towns cannot invent special setbacks for ADUs. Whatever front, side, and rear setbacks apply to your single-family house in your zoning district — those same numbers apply to your ADU. If your house can sit 10 ft off the side line, so can the ADU.
The catch: those numbers vary a lot from town to town and district to district. We keep a directory of verified town setbacks read straight from each town's ordinance — look yours up before you start sketching.
What the law didn't touch
HB 577 removed the zoning gatekeeping. It did not repeal physics or the environmental rules. Still in force: town lot-coverage limits and dimensional standards, fire separation between buildings (10 ft is the working number we plan to for a detached unit), NH DES wetland buffers (75 ft), shoreland protection (250 ft along protected waterbodies), and septic rules for lots on private wastewater. And yes — you still need a building permit. We pull it for you.
None of these are usually deal-breakers. They're site-planning constraints, and they decide where on the lot the unit goes, not whether you can build one.
What this means in practice
If you own a single-family lot in New Hampshire, the question is no longer “will the town let me?” It's “where does it fit, and what does it cost?” For the second question, our cost guide publishes every price we charge. For the first, you can drop your address into the visualizer and see a model sitting on your actual parcel, setbacks drawn — or ask us to run the check for you, free.