Where the law is
The ADU sections are RSA 674:71 (definitions), 674:72 (rules) and 674:73 (detached units), as amended by HB 577 (2025, chapter 197, effective July 1, 2025) and HB 1079 (2026, chapter 210, effective August 31, 2026). Read them on the General Court site. The NH Office of Planning and Development also publishes a summary of the HB 577 changes on nheconomy.com.
What a town must allow
- One ADU by right. In every district that allows single-family homes, one ADU, attached or detached, is allowed as a matter of right. A town can no longer send the first ADU to a special exception or a conditional use permit (674:72, I).
- A detached unit. A town must permit one detached ADU (674:73).
- No extra lot rules. No added requirements for lot size, frontage, setbacks, design review or space beyond what a single-family home needs (674:72, I).
- No ADU rules means allowed. If a zoning ordinance says nothing about ADUs, one ADU is a permitted accessory use and only building permits apply (674:72, II).
- Existing structures. A structure that existed on or before July 1, 2025, including a detached garage, can be converted even if it no longer meets setbacks or lot coverage. The town may ask for proof that it is a lawful nonconforming structure (674:72, XI, as amended by HB 1079).
- Separate electric service. A town may not deny a separate panel and service for the ADU (674:72, XII).
What a town may set
- Size. Living space may not exceed 950 sq ft unless the town allows more, and a town may not cap it below 750 sq ft (674:72, VII).
- Owner occupancy. A town may require the owner to live in one of the two units but may not say which (674:72, VI).
- Parking. Up to one extra space for the ADU where the town requires parking for the house, on site or at a dedicated off-site spot (674:72, IV). Separately, no town may require more than one residential space per unit (RSA 674:16, VII, from SB 284, effective September 13, 2025), or require that space to be garaged (HB 1588, effective September 13, 2026).
- Design. Aesthetic standards may apply to an ADU only if the town applies them to the house too (674:72, VII).
- One per home. A town need not allow more than one ADU per single-family home, and may bar them with townhouses or on rented or leased land (674:72, I).
What a town may not do
- Require that occupants be related (674:72, VIII).
- Limit an ADU to one bedroom (674:72, IX).
- Dictate how an attached ADU is entered. It needs its own entrance or access through a shared space, and the owner chooses (674:72, III).
- Ask more of the septic system than NHDES does. Water and septic must be adequate for both units under RSA 485-A:38, and a septic application goes in before construction (674:72, I and V).
In the statute's words
- By right, 674:72, I: "One accessory dwelling unit, which may be either attached or detached, shall be allowed as a matter of right."
- Detached, 674:73: "A municipality shall permit one detached accessory dwelling unit."
- Size, 674:72, VII: "The total living space of the accessory dwelling unit shall not exceed 950 square feet unless otherwise authorized by the municipality. A municipality may not restrict the total living space to less than 750 square feet."
- Owner occupancy, 674:72, VI: "A municipality may require owner occupancy of one of the dwelling units, but it shall not specify which unit the owner must occupy."
- Entry, 674:72, III: "However, the municipality shall not limit the choice of ingress and egress."
- Parking, 674:16, VII: a town "shall not require more than one residential parking space per unit or require that the parking space per unit be garaged."
- Existing structures, 674:72, XI(a): "'existing structure' means a structure existing on or before July 1, 2025, including detached garages, regardless of whether such structures violate current dimensional requirements for setbacks or lot coverage."
What the statute does not waive
Lot coverage and the other single-family rules still apply to the house and the ADU together (674:72, IV). Septic and water have to be adequate for both units under RSA 485-A:38. The state Shoreland Water Quality Protection Act (RSA 483-B) still applies within 250 feet of public waters. A building permit is still required.
Sale
The ADU cannot be sold off as a separate condominium unless the town allows it (674:72, I).
Where towns stand
Many towns rewrote their ordinances at the 2026 town meetings: Hampton, Alton, Gilford and Moultonborough among them. Others, such as Rochester and Wolfeboro, still publish older text. Where an ordinance conflicts with RSA 674:72, the statute controls. Each town page lists what the local text says and where it lags.
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