ADU rules in New Hampshire
Representative cities: Manchester, Nashua.
ADU permit rule
Permit requiredGenerally allowedcity authority
Fee basis
Set by local jurisdiction
Typical timeline
Varies
Setback
—
Area threshold
No statewide ADU mandate; an ADU is a dwelling unit and requires a building permit.
Exempt if
None — an accessory dwelling unit (in-law apartment, garage apartment) always requires a building permit and compliance with RSA 155-A.
Go / no-go
allowed
New Hampshire has no statewide by-right ADU law; whether an ADU is allowed depends on local zoning. The structure itself requires a building permit under the state residential code regardless of town ordinance.
ADU official sources: NH RSA 155-A — New Hampshire Building Code · ICC — New Hampshire State Adoptions
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Frequently asked questions
Do I need a permit to build an ADU in New Hampshire?
Per New Hampshire rules, an ADU is "yes" for a permit and the authority is "city". New Hampshire has no statewide by-right ADU law; whether an ADU is allowed depends on local zoning. The structure itself requires a building permit under the state residential code regardless of town ordinance. Always confirm with the issuing building department.
How big can an ADU be in New Hampshire?
Area threshold: No statewide ADU mandate; an ADU is a dwelling unit and requires a building permit.
What are the ADU setbacks in New Hampshire?
Setback rules vary by city in New Hampshire; confirm with the local planning or building department.
How much does an ADU permit cost in New Hampshire?
Permit fees are set locally in New Hampshire. See the official source on this page.