Check 1: Your town and your zoning district
The state law is the floor. NH RSA 674:71–73 allows one ADU by right on any single-family lot statewide, with detached units up to 950 sf. But the numbers that decide whether a footprint fits — your setbacks, your district, your paperwork — are local, and they change from town to town and district to district. So the first thing we do is pull your parcel and identify the district it sits in. We've curated the ADU rules for towns across the state — you can look up your town and see its numbers in about a minute.
Check 2: Setbacks — the same ones your house has
Here's the part of the law most people miss: your ADU's setbacks match your house's single-family setbacks. There's no separate, stricter set of lines for the ADU. Whatever front, side, and rear setbacks apply to your house in your district apply to the new unit too. We draw those lines on your parcel, and everything inside them is where an ADU can legally sit. On a small lot this is usually the check that decides which models are in play.
Check 3: Environmental buffers
Setback lines aren't the only lines on a lot. If there are mapped wetlands, we keep the building at least 75 feet away from them. If your lot touches protected shoreland, that buffer is 250 feet. Most lots have neither and skip this check entirely. But when a buffer does apply, it can carve a big bite out of the area that survived check 2 — which is why we run it before we start placing footprints, not after.
Check 4: The envelope vs. a real footprint
After checks 2 and 3, what's left is your buildable envelope — the actual shape where a building can go. Now we test real footprints against it. Our smallest model, the Barrington, needs a 16 × 24 ft pad. The Shapleigh, our flagship two-bed, is 40 × 20 ft. The Washington gets 896 sf out of a 32 × 28 ft footprint by going two stories — a deep envelope helps there, a long skinny one doesn't. One more line matters: the ADU has to sit at least 10 feet from your house and any other existing structure, for fire separation. You can drag any of our models around your own parcel with the visualizer and watch it check setbacks, wetland buffers, and fire separation live.
Check 5: Utilities and access
A footprint that fits on paper still has to be built and plumbed. So the last check is practical: if you're on septic and well, does the septic system have capacity for the added bedrooms, or does it need work? If you're on town water and sewer, where do the connections come in? Where does the trench run from the ADU to the tie-in point, and what does it cross to get there? And can a concrete truck and delivery vehicles actually reach the building spot? None of these usually kill a project — but they shape the placement, the schedule, and the quote, so we'd rather know on day one.
What to do with all this
You could pull your town's ordinance and work through the five checks yourself — the law is public and so are the maps. Or you can hand us the address and we'll do it for free. The free zoning review checks your parcel against the by-right law and your town's setbacks and tells you what fits. We won't tell you a lot qualifies until we've actually run the checks — that's the whole point of running them. And if it does fit, the cost guide shows the published all-in price for every model, so the next question has a straight answer too.