ADU Size Limits in Boston, MA
Find the maximum ADU size allowed in Boston, Massachusetts. Includes square footage limits, height restrictions, lot coverage, and bedroom count.
Boston's Local ADU Ordinance
Unlike most Massachusetts municipalities, Boston operates under its own zoning charter with ADU-specific rules that may differ from the state baseline. This means some state-level rights may not apply in the same way. Check the specific rules below for Boston's requirements.
Size Limits: Room for a Full Cottage
At 1250 sq ft, Boston allows enough space for a complete 2-bedroom accessory dwelling unit. This is well above the minimum needed for a comfortable living space and provides flexibility for a kitchen, bathroom, living area, and dedicated bedrooms.
Height and Stories
ADUs in Boston are limited to 25 feet in height and 2 stories. This allows for a two-story design, which maximizes living space on a smaller building footprint — useful if your lot has tight setbacks.
Percentage-Based Size Limit
In addition to the absolute square footage limit, Boston allows ADUs up to 75% of the principal dwelling's living area. For homeowners with larger homes, this percentage rule may allow a bigger ADU than the flat square footage cap.
Important: State Baseline Data
The information on this page reflects Massachusetts state baseline rules. Boston may have additional local requirements. We recommend confirming with your local building department before starting your project.
Boston ADU Rules at a Glance
Max Size
1250 sq ft
% of Home
75%
Max Height
25 ft
Max Stories
2
Front Setback
25 ft
Side Setback
5 ft
Rear Setback
5 ft
Parking
1 space(s)
Approval
BY RIGHT
Owner Occupancy
Required
Rental Allowed
Yes
Short-Term Rental
Not allowed
Boston Building Department
Boston Inspectional Services Department
1010 Massachusetts Ave # 5, Boston, MA 02118, USA
Phone: (617) 635-5300
Frequently Asked Questions
What is the maximum ADU size in Boston, MA?
In Boston, you can build an ADU up to 1250 square feet or 75% of your home's living area, whichever is larger. With 1250 sq ft, you have enough room for a full 2-bedroom cottage with a kitchen, bathroom, and living area. This is set by Boston's local zoning ordinance.
How tall can an ADU be in Boston?
ADUs in Boston can be up to 25 feet tall (2 stories maximum). This allows for a two-story design, which is useful for maximizing living space on a small footprint.
Is ADU size based on my home's size in Boston?
Yes. In Boston, your ADU can be up to 75% of your main home's living area or 1250 sq ft, whichever is larger. This means larger homes can support larger ADUs.
What types of ADUs are allowed in Boston?
Boston allows detached, attached, conversion ADUs. With up to 1250 sq ft allowed, a detached ADU gives you the most flexibility — enough for a full 2-bedroom cottage.
Can I add a bedroom to my ADU in Boston?
With 1250 sq ft allowed in Boston, you can comfortably fit 2 bedrooms plus a kitchen, bathroom, and living area. All bedrooms must meet building code requirements for egress windows and minimum dimensions.
How accurate is the ADU data for Boston?
The information on this page reflects Massachusetts state baseline rules under the Affordable Homes Act. Boston may have additional local requirements that differ from the state baseline. We recommend confirming with your local building department before starting your project.
Where can I find all ADU regulations for Boston?
Visit our complete ADU rules page for Boston, MA for a full overview of zoning, permits, size limits, setbacks, parking, and costs. We cover all Massachusetts municipalities with data sourced from local ordinances and state law.
Check Your Address
See exactly what ADU rules apply to your specific property in Boston.