ADU Size Limits in Somerville, MA
Find the maximum ADU size allowed in Somerville, Massachusetts. Includes square footage limits, height restrictions, lot coverage, and bedroom count.
ADU Regulations in Somerville
Somerville follows the Massachusetts state baseline under the Affordable Homes Act. This means all Massachusetts state-level ADU rights apply — including by-right approval, no owner occupancy requirement, and standardized size limits. Somerville may also have additional local regulations that supplement the state baseline.
Size Limits: Room for a Full Cottage
At 900 sq ft, Somerville 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 Somerville 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, Somerville allows ADUs up to 50% 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.
Somerville ADU Rules at a Glance
Max Size
900 sq ft
% of Home
50%
Max Height
25 ft
Max Stories
2
Front Setback
10 ft
Side Setback
3 ft
Rear Setback
3 ft
Parking
None required
Approval
BY RIGHT
Owner Occupancy
Not required
Rental Allowed
Yes
Short-Term Rental
Not allowed
Somerville Building Department
Frequently Asked Questions
What is the maximum ADU size in Somerville, MA?
In Somerville, you can build an ADU up to 900 square feet or 50% of your home's living area, whichever is larger. With 900 sq ft, you have enough room for a full 2-bedroom cottage with a kitchen, bathroom, and living area. This is guaranteed by the Affordable Homes Act.
How tall can an ADU be in Somerville?
ADUs in Somerville 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 Somerville?
Yes. In Somerville, your ADU can be up to 50% of your main home's living area or 900 sq ft, whichever is larger. This means larger homes can support larger ADUs.
What types of ADUs are allowed in Somerville?
Somerville allows detached, attached, conversion ADUs. With up to 900 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 Somerville?
With 900 sq ft allowed in Somerville, 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.
Where can I find all ADU regulations for Somerville?
Visit our complete ADU rules page for Somerville, 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 Somerville.