New Hampshire's HB 577 made one ADU by-right statewide and, new in 2025, allows detached units; the Fire Marshal labels the building.
Building approval
NH Division of Fire Safety (State Fire Marshal)
Program
NH certification label (Modular Building Program) — State insignia/seal program
ADU law
HB 577 / RSA 674:71-73 (eff. July 1, 2025) (statewide)
ADU summary
One ADU per SF lot by-right — now including detached units.
Site / structural drivers
Heavy White Mountains snow + cold
Verdict
Statewide ADU by-right (incl. detached)
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Yes. HB 577, effective July 1, 2025, amended RSA 674:71-73 so municipalities must allow one ADU per single-family lot, attached or detached, by right. The prior law effectively limited ADUs to attached units connected by an interior doorway, so the big change is that a standalone backyard ADU is now a viable, by-right option statewide. Towns can apply standards consistent with those for the primary home, but can't simply prohibit a conforming ADU.
The New Hampshire State Fire Marshal's Office, Division of Fire Safety, through its Modular Building Program (in effect since 1993). Manufacturers are approved by the office and must use an approved third-party inspection agency that reviews plans and inspects each unit; a NH certification label is affixed. The Fire Marshal audits manufacturers and agencies and also conducts on-site inspections of units being set in the state. Your local permit covers the site work.
Any modular structure relocated into or within New Hampshire must go through a relabeling process, which requires an inspector from the NH State Fire Marshal's Office to inspect the unit(s) at the site. Factor that into the timeline if you're moving an existing modular building.
General information, current as of 2026 — not legal advice. Confirm specifics with your local jurisdiction.
Snow. The White Mountains and the north country carry heavy ground-snow loads that drive roof and structural design, and a high-performance thermal envelope matters statewide for cold. Seismic risk is low. The Seacoast adds some coastal wind exposure. PSL Modular sets the snow load and envelope from your site coordinates.
Yes — these are strong cabin and short-term-rental markets, and modular places a finished, winter-ready unit quickly through a short build season. The Fire Marshal label clears the building; your work is the town/county land-use, septic, and any local STR ordinance.