Cambridge, MA

Cambridge, Massachusetts’ Accessory Dwelling Unit (ADU) Rules

City Acceptance Rating: 9/10

Last updated: 2019

What You Need to Know:

In all districts the Board of Zoning Appeal may grant a special permit for alteration of any single-family, two family, or an accessory structure in existence as of Feb 1, 2019 to provide one accessory apartment if the following conditions are met:

• The Accessory Apartment must be in the same ownership as the primary dwelling and designated for the residential occupancy.

• Prior to alteration the dwelling contains at least one thousand eight hundred (1,800) square feet of gross floor area.

• An Accessory Apartment shall not occupy more than nine hundred (900) square feet or thirty-five (35) percent of the gross floor area of the principal dwelling, whichever is less.

• Any existing two-family home may be converted to a single-family home with accessory unit by right, without need for a Special Permit.

• The exterior appearance of the accessory structure apartment may need to be compatible with the principle dwelling on the same lot and with dwellings and accessory structures on adjoining lots.

• An Accessory Structure Apartment may not exceed nine hundred (900) square feet.

• No additional parking is required for the Accessory Apartment

• The lot per dwelling unit calculation does not apply to accessory apartments or accessory structure apartments.

• Accessory apartments are exempt from FAR calculations to determine allowable gross floor area of a lot.

• Any Accessory Apartment or basement egress or stairwell may extend beyond the minimum yard regulations within the meaning of Section 5.24.2 in all districts.

Read more: ADU Amendment by Substitution