Building an ADU in New Hampshire: what state law allows

What state law lets you build

New Hampshire's ADU law is RSA 674:71 to 674:73. A town or city that adopts a zoning ordinance "shall allow accessory dwelling units in all zoning districts that permit single-family dwellings." The law adds: "One accessory dwelling unit, which may be either attached or detached, shall be allowed as a matter of right." A separate section, RSA 674:73, says a municipality "shall permit one detached accessory dwelling unit."

State law defines an ADU as a residential living unit on a lot with a single-family home that provides independent living facilities, "including provisions for sleeping, eating, cooking, and sanitation." It can be:

• Attached: within or attached to the main house
• Detached: "neither within nor physically connected to" the main house

You can build the ADU at the same time as the main house. If your town's zoning says nothing about ADUs, one ADU is automatically "a permitted accessory use, as a matter of right," and no municipal permits or conditions apply other than building permits required by statute.

Towns don't have to allow more than one ADU per single-family home. They may also ban ADUs with townhouses and other attached single-family homes, and on rented or leased land. [1][2][3]

What towns can and can't require

State law limits the rules your town can apply to one ADU:

• Lot size, setbacks, frontage and design: the town must allow one ADU "without additional requirements for lot size, ... setbacks, aesthetic requirements, design review requirements, frontage, space limitations, or other controls beyond what would be required for a single-family dwelling without an accessory dwelling unit."
• Same rules as a house: rules such as lot coverage apply to the house and ADU together, but can't be stricter than for any single-family use in the same district.
• Size: total living space "shall not exceed 950 square feet unless otherwise authorized by the municipality," and a town "may not restrict the total living space to less than 750 square feet."
• Bedrooms: a town "shall not limit an accessory dwelling unit to only one bedroom."
• Looks: aesthetic standards can apply to the ADU only if they also apply to the main house.
• Entrances: an attached ADU needs its own entrance or one through a shared space, but the town can't limit your choice.
• Occupants: a town can't require the people in the ADU to be related to the people in the main house.
• Electric service: a town "shall not deny" a separate electrical panel and service for the ADU.

New Hampshire Housing notes that many towns will need time to update their ordinances to match the 2025 changes, so check your local rules and talk with your planning office early. [1][4]

Parking

If your town requires parking for the main house, it "may require up to one additional parking space for each accessory dwelling unit." You choose whether to provide that space on your lot or at a legally dedicated off-site location.

A separate statute, RSA 674:16, VII, says a town "shall not require more than one residential parking space per unit." Since September 13, 2026, it also can't require that the parking space be garaged. [1][5]

Owner occupancy

Unlike some states, New Hampshire still lets towns require owner occupancy. A town "may require owner occupancy of one of the dwelling units, but it shall not specify which unit the owner must occupy." It may ask you to show that one of the units is your principal residence and set reasonable rules to enforce that. Check your town's ordinance if you plan to rent out both units. [1]

Septic, water and sewer

You must make "adequate provisions for water supply and sewage disposal" for the ADU under RSA 485-A:38, but the town can't require separate systems for the house and the ADU. Before you build, you submit a sewage disposal system application under RSA 485-A where it applies. A new approved system must be installed if the existing one never received construction and operating approval, or if it fails or needs repair or replacement.

A town also "may not impose greater requirements for a septic system" for a home with an ADU than the state Department of Environmental Services requires. [1]

Converting a garage or other existing building

Towns must allow ADUs in existing structures, "including but not limited to detached garages, regardless of whether such structures violate current dimensional requirements for setbacks or lot coverage." Under the current text, last amended effective August 31, 2026, an "existing structure" means one that existed on or before July 1, 2025.

If the structure doesn't meet current setback or lot coverage rules, the town may ask you to show either that it is a preexisting nonconforming structure, or that it received an earlier zoning approval or exemption. Converting it to an ADU doesn't count as a change of use that would make it lose its nonconforming status.

An ADU structure approved and built after July 1, 2025 is protected from later zoning changes if it meets the deadlines in RSA 674:39 for starting and finishing work. See our garage conversion guide for the general steps. [1]

Selling an ADU and workforce housing

You generally can't sell the ADU separately. State law says a later condominium conveyance of an ADU apart from the main house "shall be prohibited ... unless allowed by the municipality."

An ADU may count as a unit of workforce housing toward the town's regional fair share obligation if it meets the state's criteria for rental units. [1]

If your town says no

New Hampshire has a state Housing Appeals Board that hears appeals of Planning Board, Zoning Board of Adjustment, Historic District Commission and some other local decisions, as well as certain municipal permits and fees applicable to housing. It is an alternative to Superior Court for eligible appeals.

Review its filing requirements and deadlines before you file. The board's clerk is at 44 Major Wheelock Drive, Johnson Hall, Concord, NH 03301, (603) 271-1108. [6]

Where to get help

New Hampshire Housing publishes a 2026 homeowner guide, Accessory Dwelling Units: A Guide for NH Homeowners, and an ADU resource page. The guide says that under the revised law, many homeowners can "work with the local planning department, apply for permits, and build without a public hearing," but your design choices can still trigger board review if they don't meet local rules.

For the general process, see our guides to the ADU permit process, ADU costs and ADU financing. [4][7]

New Hampshire laws to know

• RSA 674:71 to 674:73 (2016 law, effective June 1, 2017)

  Requires towns and cities with zoning to allow one accessory dwelling unit with a single-family home and limits the rules they can apply to it. [1][2][3]

• 2025 amendments (Laws 2025, chapter 197, effective July 1, 2025)

  Amended RSA 674:71 to 674:73. One ADU, attached or detached, is now allowed as a matter of right, and New Hampshire Housing says many homeowners can build without the board hearings many towns used to require. [1][2][4]

• 2026 amendments (Laws 2026, chapter 210, effective August 31, 2026)

  Amended the ADU definitions in RSA 674:71 and amended RSA 674:72, including the current wording of its rule on converting existing structures such as garages. [1][3]

• RSA 674:16, VII (residential parking)

  Bars towns from requiring more than one residential parking space per unit or requiring that the space be garaged. [5]

Questions people ask