Nashua, NH Accessory Structures: ADU Rules (2026)

New Hampshire's revised ADU law (RSA 674:71 through 674:73, as amended by 2025, 197:1 — HB 577, effective July 1, 2025) requires every municipality with a zoning ordinance to permit at least one accessory dwelling unit, attached or detached, as a matter of right in all districts allowing single-family dwellings. RSA 674:71 defines an ADU as 'a residential living unit that is located on a lot containing a single-family dwelling that provides independent living facilities for one or more persons, including provisions for sleeping, eating, cooking, and sanitation, on the same parcel of land as the principal dwelling unit it accompanies.' RSA 674:72 sets the state-mandated standards: ADU size must be allowed up to 750 sq ft minimum and 950 sq ft default maximum (the municipality may authorize larger); municipalities may not impose additional lot size, setback, or aesthetic requirements beyond the underlying single-family standards; municipalities may not restrict ADUs to one bedroom; both attached and detached units are permitted; existing structures (including detached garages) may be converted to ADUs regardless of whether they violate current dimensional requirements for setbacks or lot coverage; separate electrical service must be permitted; combined septic/water systems are acceptable; one additional parking space per ADU may be required (on-site or off-site). RSA 674:73 governs owner occupancy and family rules — a municipality may require owner occupancy of one of the two units (without specifying which one), but it may NOT require that residents of the two units be related by blood, marriage, or adoption. Tiny houses meeting RSA 674:74 must be permitted as detached ADUs (a lot may have either an ADU or a tiny house, but not both). Nashua's pre-2025 Land Use Code Sec. 190-32 capped accessory dwelling units at 30 percent of the principal structure's gross floor area up to 700 sq ft, required owner occupancy, required a covenant filed with the Hillsborough County Registry of Deeds, and restricted rental to persons related by blood, marriage, or adoption to occupants of the primary dwelling. Those Nashua provisions are preempted to the extent they conflict with RSA 674:71-:73: Nashua must allow at least 750 sq ft (the 700 sq ft cap is now superseded), Nashua may not restrict ADU rental to family members (the family-relation rule is preempted), Nashua must allow detached ADUs by-right (no Special Exception required), and Nashua must allow conversion of existing garages and accessory buildings even where current setbacks would not be met. Nashua may still require owner occupancy of one unit, a covenant for recording, and adherence to the NH State Building Code (which Nashua adopts under NH RSA 155-A). Nashua's ADU ordinance is undergoing amendment to align with HB 577 — applicants should confirm current standards with the Nashua Planning Department.