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Case Studies - Accessory Dwelling Units (ADU)

View case studies conducted on the Accessory Dwelling Units (ADU) module.

Lexington, Massachusetts

Lexington is a fairly affluent community located close to Boston, with access to Routes 2 and 128. The community has an exceptionally strong school system that has fostered residential growth. Lexington is a popular tourist destination and the site of many annual events. The town lies within an area of similar communities struggling to maintain their suburban nature in the face of strong real estate market demands and growth.

Lexington is a highly built-out suburb with strong demand for housing. Lexington implemented its first accessory dwelling unit bylaw in 1983, since then only 60 units have been constructed. In February 2005, the Town amended the ADU bylaws in order to simplify the bylaws and enable the development of more accessory units. Changes made included:
- Reducing or eliminating minimum lot size or floor area requirements;
- Allowing ADUs by-right in homes built as recently as five years ago; and
- Allowing ADUs by Special Permit in new construction

Why it works:

  • Market Constraints - Land for new construction in Lexington is extremely limited and demand for housing is high. The need for accessory units will increase out of necessity; the zoning bylaws allow the existing housing market to fill this demand sustainably.
  • Clarity of Bylaws - Lexington amended its ADU bylaws in 2005 in order to improve the clarity of the provisions. The improved transparency eliminates attitudinal barriers to constructing accessory units by making the regulations clear and intuitive.
  • Flexibility - Accessory units can be constructed by-right, by special permit, or as accessory apartments in accessory structures. The clear definition of each development option expands the number of homeowners able and willing to construct accessory units.
  • Comprehensive Planning - Lexington's ADU provisions match the goals articulated in the city's comprehensive plan, which include increasing the supply of affordable housing and providing housing for middle-income households that are being priced out of the city's housing market.

Northampton, Massachusetts

Northampton is a lively, diverse community located in the heart of Pioneer Valley in western Massachusetts. The city has a strong and diverse economic base consisting of retail, commercial, and manufacturing sectors, three hospitals and Smith College, which is part of the Five-College Consortium. There is an active downtown that has numerous restaurants, clubs featuring an array of music, and street musicians.

Northampton offers a range of housing types: from apartments and condos downtown to single family homes throughout the community. In part due to Northampton’s many amenities, including its strong school system, housing prices have increased significantly. Despite this, Northampton has maintained close to 12% of their housing units as affordable. Accessory apartments offer another housing option—one that can fit the needs of the some of the region’s many students or allow an aging parent to live close to a son or daughter.

Local Accessory Dwelling Ordinance

The City of Northampton first implemented the accessory units ordinance in 1999 and had it apply to all single-family dwellings. Some of the key requirements of the ordinance include:

  • Accessory units within a single-family dwelling are by-right and if the apartment is detached, a Special Permit is required. There are no affordability restrictions required for the permitted accessory dwelling units.
  • The number of tenants is restricted to a maximum of 3 people in the accessory units, but there are no relationship restrictions.
  • Floor area of the accessory units is limited to 900 square feet.
  • There are no specific requirements on setbacks and lot coverage.
  • Three parking spaces are required on the property.
  • Access to the accessory dwelling unit must be from the side or rear yards of the primary building.
  • Units can be specified for handicapped persons.
Photos credited to Wright Builders with thanks to Northampton

    Pelham, Massachusetts

    Pelham lies within the Quabbin Reservoir watershed in central Massachusetts. The town is 31 miles north of Springfield, 85 miles west of Boston, and is situated between Routes 90 and 91. A large percentage (84%) of the homes in this largely rural community are owner-occupied. Roughly 1% of homeowners have built accessory dwelling units.

    Why it works:

    • While the ADU zoning provisions do not place affordability requirements on the accessory units, the small floor area and restricted number of tenants mandated in the bylaw create housing for a broader range of income and age levels.
    • The Town has not implemented an amnesty program for existing apartments. This ensures that units under this bylaw will meet new code requirements.