Accessory Dwelling Units in New Hampshire: What the Law Allows

New Hampshire’s accessory dwelling unit statute, RSA 674:71–73, establishes a clear statewide rule: municipalities must allow one accessory dwelling unit in all zoning districts that permit single-family dwellings. The statute is not merely permissive. It significantly restricts the ability of local zoning ordinances to interfere with a homeowner’s ability to add a second dwelling unit on an existing residential lot.

An accessory dwelling unit, or ADU, is a fully independent residential unit located on the same parcel as a single-family home and equipped for sleeping, cooking, eating, and sanitation. An ADU may be constructed at the same time as the principal dwelling or added later. It may be located within the home, physically connected to it, contained within a preexisting accessory structure such as a garage, or constructed as a fully detached, stand-alone unit.

The statute allows one ADU as a matter of right in every zoning district that permits single-family dwellings. The unit may be attached or detached, and municipalities may not impose additional lot size, frontage, setback, design review, or aesthetic requirements beyond those that apply to a single-family home without an ADU. If a town’s zoning ordinance does not address ADUs at all, the statute fills the gap automatically, and the ADU is deemed a permitted accessory use with no local approval required other than a building permit.

The legislature also addressed a number of specific issues that historically have been used to limit or discourage ADUs. The statute establishes both minimum protections for homeowners and a few outer limits that municipalities may enforce.

Key requirements and limitations

·        One accessory dwelling unit per single-family dwelling is allowed

·        The ADU may be attached or detached

·        Total living space may not be limited to less than 750 square feet and may not exceed 950 square feet unless the municipality authorizes more

·        Municipalities may not limit an ADU to a single bedroom

·        Municipalities may not require a familial relationship between occupants

·        A municipality may require owner occupancy, but may not dictate whether the owner occupies the main dwelling or the ADU

·        At most, one additional parking space may be required

·        Separate septic systems are not required, and septic standards may not exceed state requirements

·        ADUs may be created from existing structures, even if those structures do not comply with current setback or lot-coverage rules

·        Separate electrical service and panels may not be prohibited

Rentals permitted

One of the statute’s most consequential provisions is its treatment of occupancy and rentals. The law expressly prohibits municipalities from requiring a familial relationship between the occupants of the ADU and the occupants of the principal dwelling. As a result, ADUs may be rented to unrelated third parties. Although a town may require that the owner live on the property, it may not require that the owner live in one unit rather than the other. The owner may occupy either the principal dwelling or the ADU.

Detached ADUs receive essentially the same treatment as attached units. A municipality must permit one detached ADU, subject to the same substantive standards that apply to attached units. The statute removes any ambiguity on this point and prevents towns from categorically excluding detached units through zoning definitions or accessory-structure rules.

Zoning ordinances still matter

Zoning ordinances continue to apply, but only in a narrow and carefully circumscribed manner. Regulations that generally apply to single-family dwellings, such as height limits, lot coverage standards, maximum occupancy rules, and building code requirements, may be enforced so long as they are applied neutrally to the property as a whole and are no more restrictive because an accessory dwelling unit is present. Municipalities may also enforce reasonable owner-occupancy verification requirements and limited parking rules within the bounds expressly authorized by statute. What municipalities may not do is use zoning as an indirect means of reimposing restrictions the legislature deliberately removed, including enhanced dimensional requirements, special-use permits, discretionary or aesthetic review processes, or other ADU-specific controls that go beyond those applicable to single-family dwellings.

Taken as a whole, RSA 674:71-73 reflects a deliberate legislative choice to favor property-owner flexibility and modest residential density. For homeowners, the statute provides a clear entitlement to one additional dwelling unit. For municipalities, it leaves room to regulate, but little room to obstruct.

For assistance with building permits, ADUs, or other real estate matters, please contact Alfano Law at (603) 856-8411 or by filling out our Contact Form.  The firm offers free or low-cost initial consultations for most matters.