Nashua, NH Accessory Structures: Garage Conversions (2026)

A garage-to-living-space conversion in Nashua is a structural alteration and change of occupancy under the NH State Building Code (which Nashua adopts under RSA 155-A and which incorporates the 2018 IRC with NH amendments). A building permit is required, an electrical permit is required for any wiring changes, and a plumbing/mechanical permit is required if those systems are added. Code minimums for the new habitable space: ceiling height 7 ft minimum (IRC R304), light and ventilation (R303 — 8 percent glazing of floor area), egress windows from sleeping rooms (R310 — 5.7 sq ft net openable, 24 in min height, 20 in min width, 44 in max sill), interconnected smoke alarms in each sleeping room, hallway, and on each level (R314), CO alarms outside each sleeping area (R315), and NH State Energy Code insulation values. If the conversion adds cooking facilities (sink, range, refrigerator) and a separate entrance — making the converted garage an independent living unit — it becomes an Accessory Dwelling Unit. Critically, the new NH ADU framework under RSA 674:71-:73 (HB 577, effective July 1, 2025) requires Nashua to allow conversion of existing structures — including detached garages — into ADUs regardless of whether the existing structure violates current dimensional requirements for setbacks or lot coverage. That is a major change: a garage built 3 ft from the property line in 1955 may be converted to an ADU even though the current zoning requires 4 ft, and Nashua may not require a variance. Nashua's legacy Sec. 190-32 (4 ft side/rear setback for ADUs, 10 ft separation between principal and accessory dwellings, 700 sq ft cap) is preempted where it conflicts with the 750 sq ft minimum the state requires Nashua to allow. The 10 ft separation under Sec. 190-32 contains a built-in exception 'unless it would prohibit the construction of an up to 800 sq ft accessory dwelling unit' — a clue that Nashua already accommodated up to 800 sq ft in its pre-HB 577 ordinance. Parking: Nashua may require one additional off-street space for the ADU (RSA 674:72) — if the conversion eliminates the only garage parking space, a replacement driveway/paved pad may be needed.