New Hampshire RSA 674:72 requires a municipality with a zoning ordinance to allow one accessory dwelling unit (ADU), attached or detached, in zoning districts that permit single-family dwellings. If a municipality’s ordinance contains no ADU provisions, the statute deems one ADU a permitted accessory use as of right, subject to the statutory rules. Check the current state statute and local provisions for the property.
Check the existing-structure conversion rule
Effective August 31, 2026, RSA 674:72 also addresses ADUs converted from existing structures, including detached garages. For this provision, an existing structure is one that existed on or before July 1, 2025. The statute says a municipality must allow conversion even where the structure does not meet current setback or lot-coverage rules.
If a structure is nonconforming, the municipality may require the applicant to show that it qualifies as a lawful pre-existing nonconforming structure or received prior zoning approval or a determination that it was exempt from the current dimensional rules. The statute preserves the structure’s nonconforming status and says the ADU conversion is not treated as a change of use that would remove that status.
The state law sets a baseline, not a parcel outcome. Confirm the structure’s date and permit history, applicable municipal zoning, water and sewage provisions, flood or other site conditions, and any separate building permits with the local authority.