New Hamsphire ADU Laws
The Revised Law on Accessory Dwelling Units (ADUs) – 2025 Edition
Updated August 12, 2025
Introduction
On July 15, 2025, Governor Ayotte signed House Bill 577 into law, substantially amending RSA 674:71 through RSA 674:73 relative to Accessory Dwelling Units (ADUs). The legislation took effect July 1, 2025.
The revised statute establishes uniform statewide standards requiring municipalities that adopt zoning ordinances to permit Accessory Dwelling Units in all zoning districts that allow single-family dwellings.
This document provides a structured summary of the statutory requirements and limitations applicable to municipalities under the amended law.
Definition of an Accessory Dwelling Unit
An Accessory Dwelling Unit (ADU) is defined as a residential living unit 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.
An ADU may be constructed concurrently with the principal dwelling unit.
Attached Unit: A unit located within or physically connected to the principal dwelling, or completely contained within a preexisting detached structure.
Detached Unit: A unit that is neither within nor physically connected to the principal dwelling and is not contained within a preexisting detached structure.
Mandatory Municipal Requirements
1. Allowance as a Matter of Right
Municipalities shall permit one Accessory Dwelling Unit per single-family dwelling as a matter of right in all zoning districts that allow single-family residential use.
A municipality may not require a conditional use permit or special exception for the establishment of an ADU.
If a zoning ordinance contains no provisions addressing ADUs, one ADU shall be deemed a permitted accessory use to any single-family dwelling, and no municipal permits or conditions shall be required other than applicable building permits.
2. Dimensional and Regulatory Consistency
Municipalities shall not impose additional requirements relating to lot size, setbacks, frontage, aesthetic standards, design review, space limitations, or other zoning controls beyond those applicable to a single-family dwelling without an ADU.
Permissible Municipal Regulations
A municipality may adopt regulations addressing the following:
Limitation to one ADU per single-family dwelling
Prohibition of ADUs associated with multi-family dwellings (e.g., townhouses)
Prohibition of ADUs on rented or leased land
Owner-occupancy requirement for either the principal dwelling or the ADU (without specifying which unit must be owner-occupied)
Application of zoning standards applicable to single-family dwellings, including lot coverage and occupancy standards consistent with federal housing policy
Minimum and maximum size limitations, provided that an ADU shall not be limited to less than 750 square feet and shall not exceed 950 square feet unless otherwise authorized
The sale of an ADU separate from the principal dwelling through condominium conveyance is prohibited unless expressly permitted by municipal ordinance.
Attached ADUs shall have either independent ingress and egress or access through a shared common space with the principal dwelling; municipalities shall not restrict the method of access.
Prohibited Municipal Restrictions
Municipalities may not:
Require a familial relationship between occupants of the ADU and the principal dwelling
Limit an ADU to only one bedroom
Impose septic, wastewater, or water supply requirements greater than those required by the Department of Environmental Services
Require separate septic systems for the principal and accessory dwelling units
Require more than one additional off-street parking space per ADU, and only if parking is required for the principal dwelling
Deny the establishment of a separate electrical panel or separate electrical service for the ADU
ADUs in Existing Nonconforming Structures
Pursuant to RSA 674:72, XI, municipalities shall allow the conversion of existing structures, including detached garages, into Accessory Dwelling Units, even where such structures do not comply with current dimensional requirements for setbacks or lot coverage, provided the structure existed on or before July 1, 2025.
Municipalities are encouraged to consult with legal counsel regarding interpretation and implementation of this provision.