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Accessory Dwelling Unit (ADU) Rules in NH

Updated 2026-10-10 · 6 min read

NH's ADU law after the 2025 and 2026 changes: what towns must allow, what they can still require, septic approval, garage conversions and building permits.

What an ADU is under New Hampshire law

New Hampshire's accessory dwelling unit law is in RSA 674:71 through 674:73. An ADU is a separate living unit on a lot with a single-family home. It has its own space for sleeping, eating, cooking and sanitation. It can be attached (inside the house or attached to it) or detached (not physically connected to the house). The law also says an ADU can be built at the same time as the main house.

The law changed twice recently. HB 577 (2025, chapter 197) took effect July 1, 2025. It required towns to allow detached units, which they previously could choose not to allow, and it set new size rules. HB 1079 (2026, chapter 210) took effect August 31, 2026. It adjusted the definitions and the rules for converting existing buildings. This guide reflects the statute text as of October 2026.

What towns must allow

Under RSA 674:72, a town or city with a zoning ordinance must allow ADUs in every zoning district that permits single-family homes. One ADU, attached or detached, is allowed "as a matter of right." RSA 674:73 separately says a municipality "shall permit one detached accessory dwelling unit." If a town's zoning ordinance says nothing about ADUs, one ADU is a permitted accessory use for any single-family home, and the only permits that can be required are building permits.

Towns don't have to allow more than one ADU per single-family home. Town ordinances may not all have caught up with the 2025 and 2026 changes. If your town's ordinance seems out of step with the state law, ask the planning or zoning office how it is applying the changes.

What towns can still require

RSA 674:72 leaves towns some room. A town may:

Require the owner to live in one of the two units, and require proof that one of them is the owner's principal residence. The town can't dictate which unit the owner lives in.
Require up to one extra parking space per ADU, but only if it already requires parking for the main house. The owner can put that space on site or at a legally dedicated off-site spot.
Cap the ADU's living space. The default cap is 950 square feet unless the town allows more, and a town can't set the cap below 750 square feet.
Apply aesthetic standards to an ADU, but only if the same standards apply to the main house.
Apply its normal single-family rules, such as lot coverage, to the house and ADU together, as long as the ADU rules are no stricter than the rules for a single-family home in that district.
Prohibit ADUs with townhouse-style attached homes and on rented or leased land, and prohibit selling the ADU separately as a condominium unless the town chooses to allow it.

What towns can't require

Under RSA 674:72, a town can't:

Add lot size, setback, frontage, design review or other requirements for one ADU beyond what a single-family home without an ADU would face.
Require more for the septic system of a home with an ADU than NHDES requires.
Require the people in the ADU to be related to the people in the main house.
Limit an ADU to one bedroom.
Limit how an attached ADU is entered. It can have its own entrance or share a common space with the main house.
Deny a separate electrical panel and separate electrical service for the ADU.

Converting a garage or other existing building

Towns must allow ADUs to be created from existing structures, including detached garages, even if the structure doesn't meet today's setback or lot coverage rules. Under the 2026 amendment, "existing structure" means one that existed on or before July 1, 2025. The town can ask you to show that the building is a lawful preexisting nonconforming structure, or that it got an earlier zoning approval or exemption. Converting it to an ADU isn't treated as a change of use that would cost the building its nonconforming status.

This part of the law is newer and more technical than the rest, so bring your town's zoning office into the conversation early if you're converting an older garage or barn.

Septic and water: plan for this first

On a private septic system, the septic is often the deciding factor. RSA 674:72 requires adequate water supply and sewage disposal under RSA 485-A:38, but the ADU doesn't need its own separate systems. Before building an ADU, you must submit an application for septic approval under RSA 485-A. The approved system must actually be installed if your existing system never received construction and operating approval, or if it fails or needs repair or replacement.

NHDES fact sheet WD-01 says the owner has to confirm that the lot can handle the added sewage load and meet state and local setbacks. The owner must also work with a permitted septic designer to prepare and submit an Individual Sewage Disposal System application. If you're on town sewer or a public water system, the local utility's rules decide how you connect, and the fact sheet says you'll need to request a new water connection before building. For a private well, NHDES says there's no required minimum yield, and suggests having a licensed well contractor or pump installer check whether it can handle the added use.

Line up a septic designer before you finalize plans. Our septic guide covers how NH systems are approved.

Building permits and the building code

Zoning permission is only part of it. An ADU is a dwelling unit, and it has to meet the State Building Code, which now adopts the 2024 International Residential Code with NH amendments. Expect a building permit, plans and inspections as with any addition or new home. Ask your building official early what the code requires for a second unit, for example fire separation, emergency escape openings and smoke alarms. Our building permit guide explains how local permits work. Trade work in the ADU follows NH licensing rules (licensing guide).

Waterfront lots may also need state shoreland or wetlands permits. See our shoreland guide.

What the ADU law doesn't address

RSA 674:71 to 674:73 don't mention short-term rentals, lease lengths or impact fees. That doesn't mean anything goes. Other local rules may apply, so ask your town before you plan on renting the unit. If your project doesn't fit the ordinance, for example a unit larger than your town allows, you can ask the zoning board of adjustment for a variance under RSA 674:33. The board has to find that the request meets five tests, including that it isn't contrary to the public interest and that strict enforcement would cause unnecessary hardship.

Before you start

Get your town's current ADU ordinance and ask what permits, plans and parking it requires.
Have a septic designer check your system and lot, or confirm sewer and water connections with the utility.
If you're converting an older building, gather records showing when it was built and any past zoning approvals.
Get written bids from general contractors or remodelers, and follow our hiring checklist.

A note on accuracy

The ADU statute was amended in 2025 and again in 2026, and towns are still updating their ordinances. This guide is general information, not legal advice. Confirm the rules for your lot with your town's planning, zoning and building offices and with NHDES before you design or build.