In plain words

New Hampshire has a statewide ADU law. An ADU, or accessory dwelling unit, is a small, complete second home on the same lot as a house. It has its own kitchen, bathroom and sleeping space. Since July 1, 2025, every town with zoning must allow one ADU 'by right' wherever single-family homes are allowed. By right means the town cannot demand a special exception or conditional use permit. Those are case-by-case approvals that often need a hearing. You still need a building permit. The ADU can be inside your house, attached to it, or a separate building in the yard. The town cannot demand a bigger lot or extra road frontage because of the ADU. It also cannot add bigger setbacks or design review. Setbacks are the required distances from property lines. The town can require one extra parking space, but only if it requires parking for the house. Towns must allow at least 750 square feet. The default cap is 950 square feet unless your town allows more. Towns may still require 'owner occupancy.' That means the owner must live in either the house or the ADU. Towns cannot require the tenant to be a relative. If your town's zoning says nothing about ADUs, one ADU is allowed with just a building permit. A 2026 update helps you convert a garage or barn built by July 1, 2025. It can become an ADU even if it sits too close to a property line under today's rules. Since September 13, 2026, towns may also choose to allow a tiny house or yurt as the ADU. You still need approved septic or sewer capacity. Whether you can build depends on your lot, your town's ordinance and state septic approval.

The rules at a glance

State ADU lawYes
Where the state rule appliesAll municipalities that have a zoning ordinance; in towns with no ADU provisions, one ADU is automatically a permitted accessory use needing only any building permit required by statute
Approved without a hearing?Yes. Qualifying ADUs are approved by staff review, without a discretionary hearing.
Kinds of ADU the state protectsinternal, attached, detached
ADUs per lot1 ADU (attached or detached) per single-family home; towns may allow more
Owner living on siteTowns may require the owner to live on the property.
ParkingOnly if the town requires parking for the main house, it may require up to one extra space per ADU. The owner chooses whether that space is on site or at a legally dedicated off-site spot.
SizeAn ADU may not exceed 950 sq ft of living space unless the town allows more. A town may not cap ADU living space below 750 sq ft. Towns cannot limit an ADU to one bedroom.
Lot size and setbacksNo extra lot size, setback, frontage, aesthetic or design review rules beyond what a single-family home faces. Buildings that existed on or before July 1, 2025 (such as detached garages) may be converted to an ADU even if they break current setback or lot coverage rules. Towns may not require more septic capacity than the state Department of Environmental Services requires, and a separate septic system is not required.
Can towns opt out?No

What to do next

  1. Ask your town planning or building office for its current ADU ordinance; many towns rewrote theirs in 2025 and 2026 to match the new law.
  2. Find out whether your town requires owner occupancy, a recorded deed restriction, extra parking, or impact fees.
  3. Have a septic designer check whether your existing system can serve the ADU, and file the state septic application before building if one is needed.
  4. If you plan to convert a garage or barn, gather proof that it existed on or before July 1, 2025 and was legally built or approved.
  5. Apply for the building permit, and ask the utility about a separate electric meter if you want one.
  6. If you want a tiny house or yurt as your ADU, ask whether your town has adopted the 2026 option to allow them.

What your town still decides

Towns write their own ADU ordinance inside the state limits. They may require the owner to live in one of the two units (but not dictate which), may require proof that it is the owner's main residence, may apply aesthetic rules only if the same rules apply to the main house, and may ban ADUs attached to townhouses or on rented or leased land (such as many manufactured housing parks). The ADU statute says nothing about impact fees, and some cities (Manchester, for one) charge them. Towns may not require the ADU occupant to be a relative, may not deny a separate electric meter and panel, and may not limit how an attached unit is entered. Selling an ADU separately as a condominium is prohibited unless the town allows it. Every ADU still needs state-approved septic plans where applicable and any required building permit. Since September 13, 2026 (HB 1681, ch. 330), a town may choose to allow tiny houses of 600 sq ft or less, tiny houses on wheels and yurts as ADUs; this is a local option, not a requirement.

Examples of local rules

  • Manchester: Manchester's new zoning ordinance (in effect March 1, 2026) allows one ADU per single-family home: inside the house, attached, over a garage, or as a separate building. The owner must live in one of the units and record a deed restriction saying so. ADUs may have at most two bedrooms and 950 sq ft; an attached unit cannot exceed 40 percent of the house's floor area, but the rule never forces it below 750 sq ft. One extra off-street parking space is required, no new curb cut is allowed, and city impact fees apply. Source

Laws, bills and citations

The laws behind this (4)
  • Definition, inspection and local approval of tiny houses and yurts as innovative housing structures (HB 1681 (2026), Laws 2026, ch. 330, new subdivision of RSA 674 after 674:80 (Innovative Housing Structures), signed July 15, 2026, in effect September 13, 2026). Lets towns choose to allow tiny houses (600 sq ft or less, including tiny houses on wheels) and yurts, either as a home on a single-family lot or as an ADU under RSA 674:72. It is optional for towns, not a mandate, and these units must still meet building codes and local land use rules. Read it
  • Allowing accessory dwelling units to be built within or attached to certain non-conforming structures (HB 1079 (2026), Laws 2026, ch. 210, amending RSA 674:71 and 674:72, signed July 2, 2026, in effect August 31, 2026). Clarifies the 2025 rule on converting existing buildings. A structure that existed on or before July 1, 2025 (such as a detached garage) can become an ADU even if it breaks today's setback or lot coverage rules, and the town may ask the owner to show the building is legally nonconforming or was previously approved. The conversion does not count as a change of use, ADUs built after July 1, 2025 get vesting protection from later zoning changes, and a unit inside a separate existing building now counts as a detached unit. Read it
  • Relative to accessory dwelling units (detached ADUs by right) (HB 577 (2025), Laws 2025, ch. 197, amending RSA 674:71, 674:72 and 674:73, signed July 15, 2025, in effect July 1, 2025). Every town with zoning must allow one ADU, attached or detached, as a matter of right wherever single-family homes are allowed; towns can no longer require a conditional use permit or special exception. Towns cannot add lot size, setback, frontage, design review or septic rules beyond those for a single-family home, may require at most one extra parking space, and cannot cap size below 750 sq ft (the default cap is 950 sq ft unless the town allows more). Read it
  • Original statewide ADU law (SB 146 (2016), Laws 2016, ch. 6 (RSA 674:71-73), amended by Laws 2017, ch. 89 and ch. 238, signed January 1, 2016, in effect June 1, 2017). Created New Hampshire's ADU subdivision. Towns with zoning had to allow one attached ADU per single-family home, either by right, by conditional use permit, or by special exception. Detached ADUs were optional for towns until 2025. Read it
Proposed and failed bills (4). None of these is law.
  • HB 1540 (2026): died as of October 3, 2026. Started as a limit on by-right ADUs in protected shoreland areas and was narrowed to confirm that towns can enforce septic safety rules for ADUs. The Senate added unrelated liquor-license language, the two chambers never agreed, and it died in conference committee. Bill page
  • HB 1136 (2026): died as of October 3, 2026. Would have let towns require that ADU tenants be related to the owners, treat attached and detached units differently on looks, and allow more than one ADU. The House took it up on March 11, 2026 but never passed it, and it died when the 2026 session ended. Bill page
  • HB 1026 (2026): died as of October 3, 2026. Would have let towns ban ADUs built from manufactured housing. Killed in the House on March 11, 2026. Bill page
  • HB 1291 (2024): died as of October 3, 2026. Would have allowed two ADUs per home by right and raised size limits. It passed the House but the Senate killed it on May 22, 2024. Bill page

Sources

  1. HB 1681 (2026), Laws 2026, ch. 330, new subdivision of RSA 674 after 674:80 (Innovative Housing Structures) · official source
  2. HB 1079 (2026), Laws 2026, ch. 210, amending RSA 674:71 and 674:72 · official source
  3. HB 577 (2025), Laws 2025, ch. 197, amending RSA 674:71, 674:72 and 674:73 · official source
  4. SB 146 (2016), Laws 2016, ch. 6 (RSA 674:71-73), amended by Laws 2017, ch. 89 and ch. 238 · official source
  5. RSA 674:73 Detached Accessory Dwelling Units · official source · checked 2026-10-03
  6. NH Dept. of Business and Economic Affairs: HB 577 Summary of Changes · official source · checked 2026-10-03
  7. City of Manchester Zoning Ordinance (in effect March 1, 2026) · official source · checked 2026-10-03
  8. NHMA: The Revised Law on Accessory Dwelling Units, 2025 Edition (updated Aug 12, 2025) · checked 2026-10-03
  9. Citizens Count: HB 1540 (2026) · checked 2026-10-03
  10. Citizens Count: HB 1136 (2026) · checked 2026-10-03
  11. Citizens Count: HB 1026 (2026) · checked 2026-10-03
  12. Citizens Count: HB 1291 (2024) · checked 2026-10-03
  13. NH Liberty Alliance bill tracker: HB1681 (2026) text and timeline (signed July 15, 2026, ch. 330, eff. Sept. 13, 2026) · checked 2026-10-03
  14. Citizens Count: HB 1681 (2026) · checked 2026-10-03
  15. NHPR: Tiny homes, yurts explored by lawmakers as housing options (Apr 22, 2026) · checked 2026-10-03
  16. NHMA: The Revised Law on Accessory Dwelling Units, 2025 Edition (updated Nov 17, 2025) · checked 2026-10-03

This is general information, not legal advice. Your town, village, city or county decides what can be built on your property. If something here looks out of date, email the team.