ADU rules in Nashua

Detached backyard ADUs became legal in Nashua on December 23, 2025, when the Board of Aldermen passed Ord. No. O-25-076. That ordinance rewrote the Section 190-264 definition of 'Dwelling Unit, Accessory' from a unit 'contained within, or attached to the same building as a single-family dwelling' to one 'contained within, attached to, or detached from a single-family dwelling on the same lot', and it moved accessory dwelling units in the Table 15-1 Use Matrix into the Accessory Uses block with an 'A' in all fifteen zoning districts. Nashua was catching up to state law: RSA 674:73, as amended by Laws of 2025, chapter 197 effective July 1, 2025, provides that a municipality 'shall permit one detached accessory dwelling unit'. Only one ADU is allowed per lot, and it must accompany a single-family detached dwelling.

An ADU inside the house, in a basement, or in an addition attached to it has been allowed in Nashua since the city wrote Section 190-32 under the 2016 state ADU act, and Ord. No. O-25-076 kept that while adding detached units. Table 15-1 now designates accessory dwelling units 'A', a permitted accessory use, in every district including all seven residential districts (R-40, R-30, R-18, R-9, R-A, R-B and R-C). Section 190-32 B(8) is the only ingress rule: an attached ADU 'shall either have an independent means of ingress and egress or ingress and egress through a common space shared with the principal dwelling'. O-25-076 struck the old requirement that an interior door connect the two units, matching RSA 674:72, III, which now bars a municipality from limiting the choice of ingress and egress.

Section 190-32 B(1) caps the accessory dwelling unit at 980 square feet and requires it to stay 'clearly incidental and subordinate in extent, use, and purpose to the primary structure'. Subsection B(1)(a), added by Ord. No. O-25-076, raises the ceiling to 1,200 square feet where the unit is built 'within an existing structure, or a portion thereof, and no expansion of the footprint of that existing structure is proposed', which covers basement, attic and garage conversions. The same ordinance raised the base cap from 750 square feet. RSA 674:72, VII sets a statewide default of 950 square feet 'unless otherwise authorized by the municipality' and forbids a municipal cap below 750, so Nashua's 980 is a lawful local allowance above the state default. There is no ADU-specific height limit, but a detached unit is an accessory building under Section 190-31 A(1).

Section 190-32 B(5) requires that 'the owner of the property must occupy either the primary or accessory dwelling unit as the owner's principal place of residence'. It is an ongoing condition, not just a test at application: B(6) makes the owner record a covenant with the Hillsborough County registry of deeds stating that continued use of the ADU depends on compliance with all current and later ordinances, and 'the granting of a building permit for the accessory dwelling unit shall be contingent upon receipt of the covenant and fees'. Ord. No. O-25-076 left the owner-occupancy rule untouched while striking the covenant's old recital that the ADU was permitted by special exception. This survives the 2025 state reform: RSA 674:72, VI still says a municipality 'may require owner occupancy of one of the dwelling units, but it shall not specify which unit the owner must occupy'.

Section 190-32 B(7) requires 'one additional off-street parking space ... above the minimum standard for a single-family house, either on-site or at an off-site location and legally dedicated via a lease agreement or other instrument deemed acceptable by the Administrative Officer'. Table 198-1, the Parking Matrix, sets the single-family detached minimum at 2 spaces per unit, so a house with an ADU needs 3 spaces in total. Ord. No. O-25-076 added the off-site option, which tracks RSA 674:72, IV: a municipality that requires parking for the principal dwelling 'may require up to one additional parking space for each accessory dwelling unit', provided on-site or off-site 'at the property owner's discretion'.

Since Ord. No. O-25-076 an ADU is a by-right accessory use reviewed over the counter, not a Zoning Board case. The ordinance struck the old Table 15-1 row that carried an 'S' (special exception) in every residential district and inserted accessory dwelling units into the Accessory Uses block with an 'A' in all fifteen districts; the table key reads 'A means permitted only as an accessory use', and Section 190-15 C(1)(c) confines the 'S' designation to uses that need Zoning Board of Adjustment review under Section 190-134. The remaining approvals are a building permit (Building and Land Use Permit Application) plus delivery of the recorded owner-occupancy covenant and fees under Section 190-32 B(6). Note that the codified Use Matrix image on eCode360 still shows the superseded 'S' row; read the adopted ordinance. This change brought Nashua into line with RSA 674:72, I, which since July 1, 2025 requires one ADU, attached or detached, to be 'allowed as a matter of right'.

Nashua places no limit on renting the accessory dwelling unit to a long-term tenant, and RSA 674:72, VIII bars any requirement that the occupants be related to the family in the main house. The practical constraint is the owner-occupancy rule in Section 190-32 B(5): because the owner must occupy one of the two units as a principal residence, only one of them can be rented out. Section 190-32 B(9) adds that 'the single-family dwelling and the accessory dwelling unit shall not be separated in ownership (including by condominium ownership)', so the ADU cannot be sold or condominiumized away from the house, which matches RSA 674:72, I. Nashua has no short-term-rental ordinance and no rental registration or licensing program: a full-text search of the Nashua Revised Ordinances on eCode360 turns up no short-term-rental rule, and Chapter 182, Housing Standards, is a property-maintenance code only. Hotel, motel and tourist-court and lodging-house uses are prohibited outright in all residential districts under Table 15-1, and Section 190-15 C(1) provides that any use not listed is prohibited, so a homeowner planning nightly rentals should ask the Planning Department how the city classifies it.