Londonderry, New Hampshire

Londonderry, NH Accessory Dwelling Unit (ADU) Rules

City Acceptance Rating: 10/10

Last Updated: 8.28.2024

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What You Need To Know

ADUs are considered workforce housing and allowed only on properties with an owner-occupied single-family home, with a maximum of one per lot. They do not affect minimum lot size or density calculations. The accessory dwelling must be no larger than 40% of the main home's living area, with no more than two bedrooms, and it must be secondary in size and appearance.

Type of ADU Permit Required

If all requirements are met, only standard building permits are needed.

ADU Rules

• 1. Accessory to Owner-Occupied Residential Use The property to which an ADU is to be added must be owner-occupied although the owner may reside in the primary dwelling or the accessory unit. 

  2. Ownership The ADU or lot cannot be converted to a condominium or any other form of ownership other than the existing single-family dwelling. The single-family dwelling cannot be a mobile home or condominium or located in a Residential Open Space Cluster Development. 

  3. Occupancy The occupancy of the ADU cannot be utilized for short-term rentals or vacation rentals (including through sites like Airbnb and Vrbo). At a minimum the ADU must be rented out on a month-to-month basis. 

  4. Smaller than 1,000 square feet ADUs must be between 400 and 1,000 square feet including uses such as garages, porches, decks, etc. Additionally a detached ADU shall not exceed 50% of the primary residence or 1,000 square feet whichever is smaller. Detached ADUs cannot include a pool or garage if one already exists on the property. 

  5. Adequate Septic Capacity and Potable Water The property owner must provide evidence of adequate septic system design and capacity according to the standards of Stratham and the NHDES Water Division, along with adequate potable water according to the standards of the State of New Hampshire. 

  6. Two Parking Spaces Required, No New Driveway Curb Cut A minimum of 2 off-street parking spaces must be provided for both the ADU and the single family dwelling for a total of 4 parking spaces. 

  7. Compatible Exterior Design and Located in the Side or Rear Yard The architecture of the ADU must be compatible with the single-family dwelling and in character of the property, and designed so that the property retains the appearance of a single-family dwelling. Detached ADUs must have the appearance of a barn, carriage house, or other customary accessory structure. 

  8. Building and Zoning Setbacks For new detached ADUs, the ADU must be set back a minimum of 50 feet from the front property boundary. Detached ADUs must also meet structural, wetlands, and other applicable zoning setbacks. 

Resources:

https://www.londonderrynh.gov/sites/g/files/vyhlif4616/f/uploads/adz_0.pdf