The state law is a floor, not the whole rulebook. Towns may still apply reasonable local standards to ADUs — things like setbacks, lot coverage, septic capacity, building and fire codes, a size cap (as long as it allows at least 750 sq ft), and owner-occupancy of one unit. That's why the answer to "can I build one?" is statewide, but the answer to "what exactly can I build?" is town by town.
Zoning ordinances are amended at town meetings, and many municipalities haven't fully updated their posted zoning to match the 2025 state law — New Hampshire Housing itself notes that many towns will need time to align their ordinances. Even towns that have amended their ADU sections often kept language the state law now overrides. Reading only the town's posted ordinance can leave you asking permission for something that's already yours by right.
That's what this guide is for: every town page below reads the town's current ordinance side-by-side with RSA 674:72 and tells you, in plain English, which rules stand and which are superseded.
Free plain-English breakdown of your town's ADU rules under the 2025 law — plus a personalized report for your property.
Get My Free ADU Report →House Bill 577 is the 2025 New Hampshire law (2025 Chapter 197) that amended RSA 674:71–73, the state's accessory dwelling unit statute. Effective July 1, 2025, it allows one ADU — attached or detached — by right on single-family lots statewide, and it overrides local zoning provisions that conflict with it.
No. Since July 1, 2025, one accessory dwelling unit is allowed as a matter of right wherever single-family homes are permitted. A town can't require a special exception, variance, or conditional-use permit for the ADU itself — you still need a normal building permit, and town standards like setbacks still apply.
Yes. Detached ADUs are the headline change in the 2025 law: they're now allowed by right statewide. Town ordinances that prohibited detached units, or allowed them only by special exception, are superseded on that point.
There's no statewide maximum. Each town may set its own cap, but state law forbids any cap below 750 sq ft. Some towns allow considerably more. There's no state minimum size at all — the 750 sq ft figure is a limit on how strict a town can be, not a size you have to build.
A town may require the property owner to occupy one of the two units, but the owner chooses which one — the main home or the ADU. Towns can no longer require the ADU occupant to be a family member.
It depends on size, type, site prep, and utility connections. An ADU is a real house, just smaller — families typically spend between $275,000 and $400,000+ depending on the design, and ADU-specific financing options are available in New Hampshire.