ADU Rules in Portsmouth, New Hampshire

Portsmouth is one of New Hampshire's most historic and desirable cities — a compact seacoast community of approximately 22,000 on the Piscataqua River, with a celebrated 18th and 19th-century streetscape, nationally recognized restaurants, and very high real estate values. Portsmouth property owners benefit from NH's statewide ADU law while navigating a dense historic city with coastal flood zones, shoreland protections, and the important environmental legacy of nearby Pease International Tradeport.

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

New Hampshire's RSA 674:71-73 (enacted 2017, strengthened by 2021 amendments) provides the legal basis for ADU construction in Portsmouth:

• One ADU by right: Portsmouth must allow at least one accessory dwelling unit as of right on any single-family residential lot. No special permit, variance, or public hearing is required for compliant ADUs.
• Administrative approval: State law mandates ministerial review for code-compliant ADU applications.
• Owner-occupancy: The 2021 amendment (RSA 674:73) limited conditions municipalities can impose. Verify Portsmouth's current local conditions with the Planning Department.
• State law prevails: Any local ordinance provision conflicting with RSA 674:71-73 is preempted by state law.

Permitted ADU Types in Portsmouth

• Detached ADU: A standalone structure on the same lot as the primary residence
• Attached ADU: An addition to the primary home with an independent entrance
• Interior ADU: A unit created within the existing home, such as a basement apartment or upper-floor conversion
• Carriage house or garage conversion: Converting an existing accessory structure into a dwelling unit — common in Portsmouth's historic neighborhoods

Confirm with Portsmouth Planning which ADU types are permitted in your specific zoning district, especially in historic overlay areas.

General Size and Dimensional Standards

Portsmouth's compact urban fabric means lots are often small and setbacks are tight. Confirm all dimensional standards with Portsmouth Planning before designing your project.

Critical: Pease International Tradeport PFAS Contamination

Pease International Tradeport — the former Pease Air Force Base, located primarily in Newington adjacent to Portsmouth — has documented PFAS (per- and polyfluoroalkyl substances) contamination in groundwater. PFAS chemicals originated from AFFF (aqueous film-forming foam) used in firefighting training and operations at the former Air Force Base.

What this means for ADU projects:

• Properties in the Pease groundwater influence area should not rely on private wells without current water quality testing through the NH Department of Environmental Services (NHDES).
• Environmental due diligence is critical for any construction or habitation in affected areas.
• The PFAS issue is managed by NHDES and the US EPA — contact NHDES for current information on affected areas and mitigation measures.
• ADU permits in Portsmouth are processed normally, but buyers and developers of Pease-area properties should conduct full environmental site assessments.

Contact NHDES at des.nh.gov for current Pease PFAS area maps and water quality data.

Coastal and Shoreland Overlays

FEMA Coastal Flood Zones: Portsmouth's waterfront and low-lying coastal areas may be in FEMA VE (coastal high hazard, velocity wave action) and AE flood zones. Requirements in these zones include:

• VE zones: elevated construction on piles or columns, no enclosures below lowest floor
• AE zones: elevated finished floor above Base Flood Elevation, compliance with Floodplain Management Ordinance

NH Shoreland Water Quality Protection Act (RSA 483-B): Properties within 250 feet of public waters — including the Piscataqua River and tidal waters — are subject to NH's Shoreland Water Quality Protection Act. This law regulates impervious surface coverage, vegetation clearing, and structure placement within the protected shoreland buffer. ADU construction near the shoreline must account for these state-level restrictions in addition to local zoning.

Historic Districts: Portsmouth has extensive historic districts — including Strawbery Banke, South End, and the downtown core — governed by the Portsmouth Historic District Commission. Exterior modifications to structures in historic districts require approval, including new ADU entrances, additions, and detached structures. Interior conversions generally face fewer historic constraints.

Permit Process

1. Zoning and overlay verification: Confirm your lot's zoning district, historic overlay, flood zone, and shoreland designation with Portsmouth Planning.
2. Environmental check: For Pease-area properties, investigate PFAS groundwater status through NHDES before proceeding.
3. Pre-application meeting: Meet with Portsmouth Planning staff to discuss your project early.
4. Plan preparation: Work with a licensed architect familiar with Portsmouth's historic context and NH State Building Code (RSA 155-A).
5. Building permit application: Submit to Portsmouth Planning and Building Inspection.
6. Historic review (if applicable): Obtain approval from the Historic District Commission for exterior changes in designated areas.
7. Shoreland permit (if within 250 ft of public waters): Obtain NHDES shoreland permit if required.
8. Administrative ADU review: Code-compliant ADUs are reviewed without a public hearing under RSA 674.
9. Inspections and CO: Complete all inspections and receive certificate of occupancy.

Contact Information

• Portsmouth Planning Department: cityofportsmouth.com
• NH Department of Environmental Services (PFAS/Pease): des.nh.gov
• Municipal Code: library.municode.com/nh/portsmouth

Disclaimer

ADU regulations in Portsmouth are subject to change. This page is for informational purposes only and does not constitute legal, environmental, or planning advice. Always verify current requirements with the City of Portsmouth Planning Department, check NH RSA 674:71-73 for statewide ADU rights, and consult NHDES regarding Pease PFAS contamination for any Pease-area properties. Consult a licensed architect, contractor, environmental professional, or land use attorney before beginning any project.