Accessory Dwelling Unit (ADU) Rules for Southborough, MA

ADU State Law

What can you build in Massachusetts?

• –ADUs allowed by right: ADUs are permitted by right in all single-family zones across Massachusetts, except Boston, starting on February 2, 2025.
• –Applies to any residential dwelling in any zoning district that allows single-family homes, whether by-right or by special permit.
• –Municipalities may be more permissive than the state law, but cannot impose stricter requirements than those set by the state.
• –ADU size allowance: Municipalities must allow an ADU to be at least the lesser of 900 sq ft or 50% of the Gross Floor Area of the primary dwelling.
• –Municipalities can regulate the design, dimensional standards, and safety of ADUs but cannot apply stricter rules than for the principal dwelling. (But may be stricter in historic districts).
• –No minimum lot size: Municipalities cannot require a minimum lot size for the development of an ADU.
• –In a formal review, the Massachusetts Attorney General ruled that towns cannot use Floor Area Ratio (FAR) limits to restrict ADUs. Because FAR is based on lot size, and the law prohibits using lot size to limit ADUs, these local rules cannot override the state's size allowances. If the ADU meets state guidelines, it must be permitted, even if it exceeds local FAR thresholds.
• –The MA Attorney General has also disapproved of maximum footprint limits, since they conflict with the state's as-of-right allowance for ADUs up to 900 sq ft or half the main home's gross floor area.
• –Owner-occupancy is not required: Municipalities cannot require the property owner to live on the lot.
• –No family relationship requirement: Municipalities cannot require a family relationship between the occupants of the ADU and the primary home.
• –Short-term rental restrictions: Municipalities may regulate short-term rentals (e.g., Airbnb), but cannot restrict long-term rentals of ADUs (i.e. greater than 30 days).
• –Nonconforming lots or structures: Municipalities cannot prohibit the construction of an ADU just because the property is nonconforming (e.g., existing nonconforming setbacks, lot size).
• –Towns may only require one additional parking space for the ADU.
• –No parking requirements if the property is located within ½ mile of public transit (bus, train, subway, or ferry).