New Hampshire ADU law is RSA 674:71 to 674:73. HB 577 (2025 ch. 197), effective July 1, 2025, requires every town to allow one attached or detached ADU by right, caps it at 950 sq ft unless the town allows more, and bars caps below 750 sq ft. Towns may still require owner occupancy of one unit.
Last verified: October 18, 2026 against RSA 674:72 and 674:73 on the New Hampshire General Court site (gc.nh.gov), with HB 577 (2025 ch. 197) and HB 1079 (2026 ch. 210).
New Hampshire ADU law is RSA 674:71 through 674:73. Since HB 577 (2025 ch. 197) took effect July 1, 2025, every town with zoning has to allow one ADU, attached or detached, by right wherever single-family homes are allowed. The default size cap is 950 square feet unless the town allows more, and no town can cap an ADU below 750 square feet. Towns may still require the owner to live in one of the two units.
Last verified: October 18, 2026, against RSA 674:72 and RSA 674:73 on the New Hampshire General Court site. Subsections marked “Justia 2025 codification” in the table were read from that codification and not re-read on gc.nh.gov for this update.
What does New Hampshire ADU law allow?
New Hampshire ADU law is RSA 674:71 through 674:73. By right means the use is allowed without a special exception or discretionary permit. Under RSA 674:72, I, a municipality that adopts zoning must allow accessory dwelling units in all zoning districts that permit single-family dwellings, and one ADU, which may be either attached or detached, must be allowed as a matter of right. An attached ADU is a unit within or physically connected to the principal dwelling, or completely contained within a preexisting detached structure. A detached ADU is a unit that is neither within nor connected to the principal dwelling nor contained within a preexisting detached structure.
The by-right allowance comes with no extra dimensional hurdles. RSA 674:72, I bars additional requirements for lot size, setbacks, frontage, space limitations, or other controls beyond what would be required for a single-family dwelling. A municipality is not required to allow more than one ADU per single-family dwelling. If a zoning ordinance is silent on ADUs, RSA 674:72, II provides that one ADU is deemed a permitted accessory use as a matter of right.
| Rule | New Hampshire standard | Statute | Source |
|---|---|---|---|
| By-right ADU | One ADU, attached or detached, allowed as a matter of right where single-family homes are allowed | RSA 674:72, I | gc.nh.gov (checked October 2026) |
| Detached ADU | A municipality shall permit one detached ADU | RSA 674:73 | gc.nh.gov (checked October 2026) |
| Zoning silence | If the ordinance is silent, one ADU is a permitted accessory use by right | RSA 674:72, II | Justia 2025 codification |
| Size ceiling | Total living space shall not exceed 950 sq ft unless the municipality authorizes more | RSA 674:72, VII | gc.nh.gov (checked October 2026) |
| Size floor | A municipality may not restrict living space below 750 sq ft | RSA 674:72, VII | gc.nh.gov (checked October 2026) |
| Owner occupancy | A municipality may require owner occupancy of one unit, but not specify which | RSA 674:72, VI | gc.nh.gov (checked October 2026) |
| Parking | Up to one additional space per ADU, owner chooses on-site or off-site | RSA 674:72, IV | Justia 2025 codification |
| Bedrooms | A municipality shall not limit an ADU to only one bedroom | RSA 674:72, IX | Justia 2025 codification |
| Family relationship | A municipality shall not require a familial relationship between occupants | RSA 674:72, VIII | Justia 2025 codification |
| Conversions | Towns shall allow ADUs in or attached to existing structures, including nonconforming ones built on or before July 1, 2025 and detached garages (as amended by HB 1079, effective August 31, 2026) | RSA 674:72, XI | gc.nh.gov (checked October 2026) |
How big can an ADU be in New Hampshire?
RSA 674:72, VII sets the size rules. The total living space of an ADU shall not exceed 950 square feet unless otherwise authorized by the municipality. That 950 square foot default ceiling was added by the 2025 expansion. At the same time, a municipality may not restrict the total living space to less than 750 square feet, so 750 square feet is the floor a town must allow. A town may not limit an ADU to only one bedroom under RSA 674:72, IX.
Does New Hampshire require owner occupancy?
New Hampshire expressly permits a municipality to require owner occupancy. RSA 674:72, VI states that a municipality may require owner occupancy of one of the dwelling units, but it shall not specify which unit the owner must occupy. This is the single biggest way New Hampshire diverges from California: California prohibits an owner-occupancy mandate on an ADU, while New Hampshire allows a town to impose one. If you plan to rent out the property in New Hampshire, check whether your town has adopted an owner-occupancy requirement.
What can New Hampshire towns still regulate?
New Hampshire leaves several controls with local governments, within the limits set by RSA 674:72.
- Parking: if parking is required for the principal dwelling, a town may require up to one additional parking space per ADU, provided on-site or off-site at the property owner’s discretion (RSA 674:72, IV).
- Owner occupancy: a town may require an owner to occupy one of the two units, but cannot dictate which one (RSA 674:72, VI).
- Water and sewer: an applicant must make adequate provisions for water supply and sewage disposal, but separate systems shall not be required for the principal and accessory units (RSA 674:72, V).
- Ingress and egress: attached ADUs need independent egress or shared common-space egress, and the municipality shall not limit the choice of ingress and egress (RSA 674:72, III).
A town may not require a familial relationship between occupants (RSA 674:72, VIII), may not limit an ADU to one bedroom (RSA 674:72, IX), and may not deny a separate electrical panel and separate electrical service to the ADU (RSA 674:72, XII).
What changed with the 2025 expansion (HB 577)?
For years the core rule was RSA 674:71 through 674:73, originally enacted by SB 146 (2016 ch. 6), which required one ADU by right where single-family homes are allowed. HB 577 (2025 ch. 197), effective July 1, 2025, substantially expanded it.
HB 577 made four main changes:
- Detached ADUs are now allowed by right. RSA 674:72, I now guarantees one ADU that may be either attached or detached, and RSA 674:73 now says a municipality shall permit one detached ADU. Previously the by-right guarantee centered on attached units and detached units were at municipal option.
- It added the attached and detached definitions to RSA 674:71, II and III.
- It set the 950 square foot default size ceiling in RSA 674:72, VII, while preserving the rule that towns may not cap below 750 square feet.
- It added conversions of existing structures (RSA 674:72, XI) and separate electrical service (RSA 674:72, XII).
How New Hampshire compares to other states
New Hampshire guarantees one ADU by right, attached or detached, and lets towns require owner occupancy. That owner-occupancy posture is the opposite of California’s. For a neighboring New England comparison, see our Maine ADU law page and our Massachusetts ADU law page. If you are weighing a small detached build against a movable structure, see our tiny house in the backyard page. You can browse every state in the ADU laws library.
What changed in 2026?
HB 1079 (2026 ch. 210) took effect August 31, 2026. It amends RSA 674:72, XI so an ADU can be built within or attached to an existing nonconforming structure, as long as that structure was built on or before July 1, 2025, and that includes a detached garage. In plain terms: an older garage that sits too close to the lot line, or a barn that breaks today’s lot coverage rule, can now hold an ADU without a variance just because the building itself is nonconforming. Confirm how your town applies it before you design around it.
What changes in 2027?
No further statewide New Hampshire ADU changes are scheduled as of October 18, 2026. HB 1079 (Chapter 210), effective August 31, 2026, is the most recent enacted change. We recheck the New Hampshire Legislature each session and will update this page when a new bill is chaptered.
Frequently Asked Questions
Does New Hampshire allow ADUs?
Yes. Under RSA 674:72, any municipality with zoning must allow one ADU as a matter of right in every zoning district that permits single-family homes. If a town’s ordinance is silent on ADUs, one ADU is deemed a permitted accessory use by right.
Are detached ADUs allowed in New Hampshire?
Yes, by right, since HB 577 (Chapter 197) took effect July 1, 2025. RSA 674:73 says a municipality shall permit one detached accessory dwelling unit. Before HB 577, detached ADUs were at municipal option.
Can a New Hampshire town require owner occupancy for an ADU?
Yes. RSA 674:72, VI expressly permits a town to require owner occupancy of one of the two units, though the town cannot dictate which unit the owner lives in. This is the opposite of California, which prohibits owner-occupancy mandates.
How big can an ADU be in New Hampshire?
The default cap is 950 square feet unless the town authorizes larger, and a town may not restrict an ADU to under 750 square feet, under RSA 674:72, VII.
How many parking spaces are required for an ADU in New Hampshire?
At most one additional space per ADU, and only if the town already requires parking for the main house. The owner chooses whether to provide it on-site or off-site, under RSA 674:72, IV.
Can a New Hampshire town require a family relationship or limit an ADU to one bedroom?
No to both. RSA 674:72, VIII bars a familial-relationship requirement between occupants, and RSA 674:72, IX bars limiting an ADU to only one bedroom.
Can I convert a garage into an ADU in New Hampshire?
Yes. Since HB 1079 (2026 ch. 210) took effect August 31, 2026, RSA 674:72, XI lets you build an ADU within or attached to an existing structure, including a detached garage, even if the structure is nonconforming, as long as it was built on or before July 1, 2025.
Sources
- New Hampshire RSA 674:72 (New Hampshire General Court, checked October 2026): https://gc.nh.gov/rsa/html/LXIV/674/674-72.htm
- New Hampshire RSA 674:73 (New Hampshire General Court, checked October 2026): https://gc.nh.gov/rsa/html/LXIV/674/674-73.htm
- New Hampshire RSA Chapter 674 table of contents: https://gc.nh.gov/rsa/html/NHTOC/NHTOC-LXIV-674.htm
- New Hampshire RSA 674:72 (Justia 2025 codification, for the subsections marked in the table): https://law.justia.com/codes/new-hampshire/title-lxiv/chapter-674/section-674-72/
- New Hampshire Housing, ADU guidance: https://www.nhhousing.org/housing-challenges-solutions/accessory-dwelling-units/
Disclaimer
This page is general information, not legal advice. ADU law changes often and towns apply it differently, so confirm the current rules with your local planning department before you build.