Concord, New Hampshire

Concord, NH Accessory Dwelling Unit (ADU) Rules

City Acceptance Rating: 6/10

Last Updated: 8.28.2024

Image Source: Visit Concord

What You Need To Know

The purpose of allowing accessory dwelling units is to enhance Alton’s affordable housing options and make better use of existing homes, while preserving the town's rural, one-family residential character. This initiative aims to provide small rental units that support low-income families and improve the overall housing inventory.

Type of ADU Permit Required

By Special Exception if following requirements are met..

ADU Rules

• Subordination and Aesthetics: The ADU must be clearly subordinate to the main residence and maintain the aesthetic continuity of a single-family home.

• One Per Lot: Only one ADU is allowed per lot.

• Not Considered Additional Unit: The ADU does not count as an additional dwelling unit for lot size or density calculations.

• Location: The ADU must be within or attached to a single-family home or within an accessory structure on the same property. No ADU is allowed on lots with more than one dwelling unit.

• Lakeshore Residential District: ADUs in this district are only allowed within or attached to a single-family home and require a Special Exception from the Zoning Board of Adjustment. Specific criteria in SECTION 413 do not apply to ADUs in this district.

• Bedroom Limit: The ADU can have no more than two bedrooms.

• Size Limits:

• Maximum of 1,500 sq. ft. of total floor area.

• Must not exceed 35% of the total finished floor space of the main dwelling.

• If in an accessory structure, it must not exceed 50% of the structure's floor space or 35% of the main dwelling's finished floor space, whichever is less.

• Minimum size of 750 sq. ft. is required.

• Entrances and Exits: Any new entrances or exits for the ADU must be located on the side or rear of the main dwelling.

• Parking: A minimum of four off-street parking spaces must be provided for the main residence and the ADU.

• Sewage Disposal: Prior to constructing an ADU, the sewage disposal system must be approved and meet NH RSA 485-A standards. An approved system must be installed if the existing one is not compliant or needs replacement.

• Owner Occupancy: Either the main residence or the ADU must be occupied by the property owner.

• Interior Door: An interior door must connect the main residence and the ADU if the ADU is within or attached to the main dwelling.

• Ownership: The single-family home, ADU, and lot cannot be converted into separate legal ownerships, such as condominiums.

• Prohibited with Certain Dwellings: ADUs are not allowed with multiple single-family dwellings, such as townhouses, or with manufactured housing as defined by NH RSA 674:31.Accessory Dwelling Unit (ADU): A residential unit within or attached to a single-family detached dwelling, providing independent living facilities (sleeping, eating, cooking, and sanitation) on the same property as the main dwelling. A house with an ADU counts as two dwelling units.

  1. Permitted Districts: ADUs are allowed by Special Exception in specific districts: RO, RM, RS, RN, RD, RH, CN, CVP, IS, and UT.

  2. Size: ADUs must be between 600 and 750 square feet.

  3. Bedrooms: ADUs can have a maximum of two bedrooms.

  4. Connectivity: ADUs must share a common wall with the main house and have at least one fire-rated interior door connecting the units.

  5. Lot Size: The property must meet the minimum lot size requirements for its district.

  6. Dimensional Requirements: ADUs must comply with all dimensional requirements (frontage, yards, lot coverage, building height) unless the existing building is non-conforming.

  7. Design Guidelines: Any exterior modifications requiring a Building Permit are subject to Development Design Guidelines.

  8. Ownership: The property owner must reside in either the main house or the ADU.

  9. Occupancy: The ADU must be used as a principal residence, not for short-term rentals.

  10. Condominium Conversion: The property cannot be converted to a condominium.

  11. Parking: A minimum of two parking spaces are required, plus one additional space per ADU bedroom.

  12. Address: The ADU must have a separate address and mail receptacle.

  Other Requirements:

  1. Fire Separation: Fire separation between units is mandatory.

  2. Septic Tanks: Separate septic tanks are required if municipal sewer is unavailable, but common leach fields are permitted.

  3. Utilities: Separate utilities are not required

Resources:

https://www.concordnh.gov/DocumentCenter/View/18490/ADU-Requirements-docx