Lexington, MA

Lexington, MA Accessory Dwelling Unit (ADU) Rules

City Acceptance Rating: 7/10

Last updated: 11/4/2025

Image source: Yankee Magazine

What You Need To Know

Lexington, like all of Massachusetts since the passage of the 2024 Affordable Homes Act, allows ADUs by right. Each town has slightly different rules surrounding ADUs, but they all must adhere to the framework outlined by the state. There are two pathways to building an ADU in Lexington.  A Protected Use Accessory Dwelling Unit (PADU) and an Accessory Apartment are the two pathways to building.  Each has specific regulations associated with it and are detailed below.

Type of ADU Permit Required

A PADU does not require a special permit and can be built using a traditional building permit, while an accessory apartment requires a special permit. 

ADU Rules

• Protected Use ADU Regulations:

  • An ADU can be maximum 900 square feet, or half the Gross Floor Area of the main house; whichever is smaller. 

  • There are no minimum lot size requirements for building an ADU. 

  • ADUs can be rented, but may not be able to be rented short-term. 

  • The property owners are not required to live on site.

  • The property owners are allowed to build one ADU on their property.

  • The ADU may be attached or detached to the primary dwelling. 

  • One additional parking space (on-street or off-street) is required if the property is further than 0.5 miles from a transit station. 

  General requirements for Accessory Apartments in Lexington:

  • There is only one ADU permitted per lot.

  • The owner of the main house must live in either the primary dwelling or the ADU.

  • If the owner is away from the property for less than two years, both the main house and ADU may be rented as long as:

  • Notice is given to the Building Commissioner

  • The owner must reside on the property at least two years before and after such leave

  • The ADU must be have a similar architectural style to the single family dwelling and meet the following criteria:

  • No exterior stairways.

  • Only one door can have the appearance of a principal entrance.

  Lexington distinguishes between three different ADUs

  • Basic ADU

  • Maximum of 1,000 square feet for gross floor area

  • There cannot be greater than two bedrooms

  • The ADU must be located in the main house

  • Expanded ADU

  • The gross floor area cannot be greater than 40% of the gross floor area of the main house

  • The ADU is designed so that it is consistent with the appearance of a single family home

  • The ADU must be located in the main house

  • Accessory Structure ADU

  • This is an ADU in an accessory structure, i.e. a garage

  • Maximum of 1,000 square feet for gross floor area

  • The ADU is designed to have a similar architectural style of the single family home on that lot and surrounding lots

Resources:

https://www.lexingtonma.gov/2226/Accessory-Dwelling-Units-ADUs

https://www.lexingtonma.gov/DocumentCenter/View/14679/760-CMR-7100-Protected-Use-ADUs---Final