ADU Rules in Merrimack, New Hampshire

Merrimack is a prosperous suburban town in Hillsborough County, with a population of approximately 27,000. Located between Nashua and Manchester along the Merrimack River, the town is best known as the home of the Anheuser-Busch brewery — one of New Hampshire's largest employers and a major regional attraction. Merrimack is a quintessential southern NH suburb, with strong I-93 access and a large population of Boston-area commuters. Despite its size and suburban character, Merrimack is governed as a town by a Select Board under the New England town meeting tradition. Property owners in Merrimack benefit from New Hampshire's statewide ADU mandate.

New Hampshire Statewide ADU Law (RSA 674:71-73)

New Hampshire's RSA 674:71-73 (enacted 2017, strengthened by 2021 amendments) provides the right to build ADUs in Merrimack:

• One ADU by right: The Town of Merrimack must allow at least one accessory dwelling unit as of right on any single-family residential lot. No Town Meeting vote, variance, or public hearing is required to exercise this right.
• Administrative approval: ADUs meeting local dimensional standards must be approved administratively.
• Owner-occupancy: The 2021 amendment (RSA 674:73) restricted conditions municipalities can impose. Verify Merrimack's current local conditions with the Planning Department.
• State law prevails: Any Merrimack local ordinance provision conflicting with RSA 674:71-73 is superseded by state law.

Permitted ADU Types in Merrimack

• Detached ADU: A standalone accessory structure on the same lot as the primary single-family home
• Attached ADU: An addition to the primary home with an independent entrance
• Interior ADU: A unit created within the existing home footprint, such as a basement apartment or in-law suite
• Garage conversion: Converting an attached or detached garage into a self-contained dwelling unit

Contact Merrimack Planning & Zoning to confirm which ADU types are permitted in your specific zoning district.

General Size and Dimensional Standards

Confirm current dimensional standards with Merrimack Planning & Zoning before designing your ADU.

Special Overlay Considerations

Merrimack River Flood Zones: The Merrimack River flows along the eastern boundary of the town. Properties near the river may be in FEMA AE or other Special Flood Hazard Areas. ADUs in these zones must comply with Merrimack's Floodplain Management Ordinance, requiring elevated construction and restricting habitable basements.

Industrial and Commercial Zones: Merrimack has significant industrial and commercial land along the Route 3 and Daniel Webster Highway corridors. Properties in these zones are not covered by RSA 674's by-right residential ADU mandate. Verify your parcel's residential zoning before proceeding.

Suburban Lot Character: Merrimack's residential neighborhoods are predominantly single-family suburban, with larger lot sizes than older NH cities. This typically means more room for detached ADUs than in denser urban areas, but setback and lot coverage requirements still apply. Verify your lot's dimensional standards with Planning & Zoning.

NH Lakes and Wetlands: Portions of Merrimack contain smaller ponds and wetland areas. Properties near these water bodies may have additional setback or impervious surface restrictions under NH DES wetland rules.

Town Governance

Merrimack's zoning ordinance is adopted and amended through Town Meeting:

• RSA 674's ADU mandate already gives property owners the right to build ADUs regardless of local ordinance text.
• The Merrimack Planning Board handles site plan and subdivision review.
• The Zoning Board of Adjustment (ZBA) hears variances and appeals.

Permit Process

1. Zoning verification: Confirm your lot's zoning district and any overlays with Merrimack Planning & Zoning.
2. Pre-application consultation: Discuss your project with planning staff before committing to a design.
3. Plan preparation: Work with a licensed architect or designer; plans must conform to NH State Building Code (RSA 155-A, IBC/IRC based).
4. Building permit application: Submit to Merrimack Building Inspection / Planning & Zoning Department.
5. Administrative review: Code-compliant ADUs receive administrative approval under RSA 674 — no public hearing required.
6. Inspections: Schedule and pass all required construction inspections.
7. Certificate of occupancy: Obtain CO before occupying the ADU.

Contact Information

• Town of Merrimack Planning & Zoning: merrimacknh.gov
• Municipal Code: library.municode.com/nh/merrimack

Disclaimer

ADU regulations in Merrimack are subject to change. This page is for informational purposes only and does not constitute legal or planning advice. Always verify current requirements with the Town of Merrimack Planning & Zoning Department and review NH RSA 674:71-73 for statewide ADU rights. Consult a licensed architect, contractor, or land use attorney before beginning any project.