ADU Rules in Manchester, New Hampshire

Manchester is New Hampshire's largest city, with a population of approximately 115,000 and a rich history as a 19th-century mill city along the Merrimack River. Property owners in Manchester benefit from New Hampshire's strong statewide ADU law while navigating local zoning rules, Merrimack River flood zones, and Manchester-Boston Regional Airport (MHT) overlay zones.

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

New Hampshire enacted one of the earlier statewide ADU mandates in the Northeast with RSA 674:71-73 (effective 2017, strengthened by 2021 amendments). Key provisions include:

• One ADU by right: Every municipality must allow at least one accessory dwelling unit as of right on any lot containing a single-family dwelling in a residential zone.
• No owner-occupancy requirement: The 2021 amendment (RSA 674:73) removed mandatory owner-occupancy conditions, though municipalities may still impose conditions on ADUs that are reasonable and consistent with state law.
• Streamlined approval: ADUs meeting local dimensional standards must be approved administratively — no public hearing or discretionary review is required.

Manchester's local zoning ordinance incorporates these state requirements. Where local rules conflict with RSA 674:71-73, state law controls.

Permitted ADU Types in Manchester

• Detached ADU: A standalone accessory structure on the same lot as the principal single-family dwelling
• Attached ADU: An addition to the primary home with a separate entrance
• Interior/Basement ADU: A unit created within the existing footprint of the home, including basement conversions
• Garage conversion: Converting an attached or detached garage into a dwelling unit

Verify with the Manchester Planning & Community Development Department which ADU types are permitted in your specific zoning district.

General Size and Dimensional Standards

Always confirm current dimensional standards directly with Manchester Planning, as local rules are updated periodically.

Special Overlay Considerations

Airport Noise and Approach Zones (MHT): Manchester-Boston Regional Airport generates noise and safety overlay zones affecting residential properties in the eastern portions of the city and surrounding areas. These overlays may limit habitable floor area, require soundproofing, or restrict new residential construction. Verify your parcel's airport overlay status before designing an ADU.

Merrimack River Flood Zones: Much of Manchester's riverfront and low-lying areas are in FEMA Special Flood Hazard Areas (AE zones). ADUs in these areas must comply with the city's Floodplain Management Ordinance, which typically requires finished floors to be elevated above the base flood elevation and restricts below-grade habitable space.

Historic Mill District: Some areas near the Millyard are subject to historic preservation review. Exterior changes to structures in historic overlay zones may require approval from the Manchester Historic District Commission.

Permit Process

1. Confirm zoning: Verify your lot's zoning district and overlay zones using Manchester's zoning map or by contacting the Planning Department.
2. Pre-application meeting: Schedule an optional pre-application consultation with Manchester Planning & Community Development staff.
3. Prepare plans: Engage a licensed designer or architect to prepare site plans, floor plans, and elevations meeting NH State Building Code (RSA 155-A, based on IBC/IRC).
4. Submit application: File a building permit application with the Manchester Building Inspection Department, including all required drawings and documents.
5. Review and approval: Administrative review for code-compliant ADUs. Flood zone or historic overlay properties may require additional review steps.
6. Inspections: Schedule required inspections during and after construction.
7. Certificate of occupancy: Obtain CO before occupying the ADU.

Contact Information

• Manchester Planning & Community Development: (603) 624-6450 | manchesternh.gov
• Manchester Building Inspection: Contact via the main city line above
• Municipal Code: library.municode.com/nh/manchester

Disclaimer

ADU regulations in Manchester 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 City of Manchester Planning & Community Development 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.