ADU Regulations in New Hampshire (NH)
New Hampshire is rated Moderate for ADU construction with an ADU-friendliness score of 55/100 (ADU-Friendly). New Hampshire has a statewide ADU law that sets baseline rules across all municipalities. Owner must reside on the property. Additional parking may be required. New Hampshire has required all municipalities to allow ADUs since 2017 (RSA 674:71-73). However, municipalities may impose reasonable conditions including owner occupancy, parking, and size limits. Many towns have adopted varying local conditions. The law was strengthened in 2024 to limit municipal restrictions.
Full ADU Regulations Summary for New Hampshire
| ADUs Allowed | Yes |
| Statewide ADU Law | Yes |
| Maximum ADU Size | 800 sq ft |
| Owner Occupancy Required | Yes |
| Parking Required | Yes |
| Minimum Setback | Set locally |
| Permitting Timeline | Varies by municipality |
| Impact Fees | Varies by municipality |
| Long-Term Rental Allowed | Yes |
| Short-Term Rental (Airbnb) Allowed | Yes |
| Ease of Building | Moderate |
ADU Regulations by City in New Hampshire
ADU rules can vary by city even within the same state. While New Hampshire's statewide law sets minimum standards, some cities have additional local provisions. Below are ADU details for 3 cities in New Hampshire.
Manchester
ADUs allowed per state law. Manchester permits attached and detached ADUs with owner occupancy. Maximum 800 sq ft.
Nashua
ADUs permitted in residential zones. Nashua has moderate ADU restrictions within state law framework.
Concord
ADUs allowed with owner-occupancy requirement. Concord has seen moderate ADU construction.
New Hampshire's ADU Law After HB 577 (July 1, 2025)
New Hampshire was an early ADU state — RSA 674:72 has required towns to allow accessory dwelling units since June 1, 2017. But the 2017 law let towns choose between by-right and special-exception treatment and made detached ADUs optional. HB 577 (Chapter 197, Laws of 2025), effective July 1, 2025, rewrote the statute:
- One ADU — attached or detached — as a matter of right in every zoning district that permits single-family homes, in every municipality with zoning. No special exception, no conditional use permit.
- Size floor and ceiling: towns cannot cap ADU living space below 750 sq ft; the statute's default maximum is 950 sq ft unless the town allows more. No one-bedroom limits, and no familial-relationship requirements ("in-law apartment" restrictions are gone).
- No extra zoning hurdles: no additional lot size, frontage, setback, or aesthetic/design-review requirements beyond what a plain single-family home faces (RSA 674:73 now says a municipality shall permit one detached ADU).
- Parking: at most 1 additional space, on-site or dedicated off-site at the owner's choice.
- Conversions protected: converting an existing structure (a detached garage, a barn) must be allowed even if it sits inside today's setbacks or exceeds lot coverage. Utilities: a separate electrical panel or service for the ADU can't be denied, and towns can't impose septic rules stricter than NHDES standards.
- What towns kept: owner-occupancy — a town may still require the owner to live in one of the two units. This is the big difference from Massachusetts and Colorado, which ban that condition.
State Law vs. Your Town's Ordinance: Check the Date
Many NH town ordinances were written for the 2017 law and haven't caught up. Londonderry's ordinance (§ 5.18, adopted 2015) is a good example of the older pattern: owner-occupancy, a cap of 40% of the principal dwelling's living area, two-bedroom max, and design-matching requirements (town permit application). Where a pre-2025 local rule is stricter than the amended statute — for example, a percentage cap that lands below the 750 sq ft state floor on a modest home, or aesthetic requirements beyond single-family standards — the state statute sets the floor. If your town's published rules predate July 2025, ask the planning department directly how they're applying RSA 674:72 as amended; ordinances in Concord, Bedford, and similar towns are in various stages of catching up.
Practical path: confirm your lot's septic (or sewer) capacity for a second unit → check the town's current ADU section and its adoption date → if the ordinance conflicts with RSA 674:72 as amended by HB 577, raise it with planning staff before redesigning your project → expect at most 1 added parking space and a building permit process, not a zoning hearing.
Sources
- RSA 674:72 — Accessory Dwelling Units (current text)
- RSA 674:73 — Detached Accessory Dwelling Units
- HB 577 (2025), Chapter 197 — chaptered act text
- Londonderry, NH — ADU permit application (example town process)
Statutes and town documents last reviewed July 2026.
How New Hampshire Compares to Other States
Below is a comparison of New Hampshire with five states that have similar ADU-friendliness scores. This comparison helps illustrate where New Hampshire stands nationally.
| State | Score | Statewide Law | Parking Req? | Owner Occ? | Ease |
|---|---|---|---|---|---|
| New Hampshire (NH) | 55 | Yes | Yes | Yes | Moderate |
| New Mexico (NM) | 55 | No | No | No | Moderate |
| Utah (UT) | 50 | Yes | Yes | Yes | Moderate |
| Rhode Island (RI) | 63 | Yes | No | Yes | Moderate |
| Alaska (AK) | 45 | No | Yes | No | Moderate |
| New Jersey (NJ) | 66 | Yes | No | Yes | Moderate |
Frequently Asked Questions About ADUs in New Hampshire
Can I build an ADU in New Hampshire?
Yes, ADUs are allowed in New Hampshire. New Hampshire has a statewide ADU law that requires local jurisdictions to permit ADUs on residential lots. This means your city cannot outright ban ADUs. The ease of building is rated "Moderate" with a friendliness score of 55/100.
Do I need to live on the property to have an ADU in New Hampshire?
Yes, New Hampshire generally requires owner occupancy — the property owner must live either in the main house or the ADU. This means you cannot rent out both units simultaneously.
How big can an ADU be in New Hampshire?
The state maximum ADU size in New Hampshire is 800 square feet. Local jurisdictions may set lower limits. Some cities also limit ADU size to a percentage of the primary dwelling (e.g., 50%).
Do I need extra parking for an ADU in New Hampshire?
New Hampshire may require additional off-street parking for ADUs. The specific number of spaces depends on your local jurisdiction. This can be a significant barrier if your lot has limited space for parking.
Can I rent out my ADU in New Hampshire?
Yes, long-term rentals of ADUs are allowed in New Hampshire. Short-term rentals (like Airbnb) are also generally permitted, though your city may have separate short-term rental regulations.
How long does it take to get an ADU permit in New Hampshire?
The permitting timeline in New Hampshire is: Varies by municipality. The statewide law sets maximum review periods that cities must comply with. Actual timelines can be affected by plan review backlogs, inspection scheduling, and the complexity of your project. Pre-approved ADU plans, where available, can significantly speed up the process.
Check ADU Rules for Your City
Use our ADU regulations lookup tool to find the specific rules for your city in New Hampshire, or browse all 50 states to compare ADU regulations across the country. Remember that local rules may be more specific than state-level regulations.