ADU Size Limits in Chelmsford, MA
The maximum ADU size in Chelmsford is 900 sq ft, or 50% of your home.
Find the maximum ADU size allowed in Chelmsford, Massachusetts. Includes square footage limits, height restrictions, lot coverage, and bedroom count.
ADU Regulations in Chelmsford
Chelmsford 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. Chelmsford may also have additional local regulations that supplement the state baseline.
Size Limits: Room for a Full Cottage
At 900 sq ft, Chelmsford 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.
Percentage-Based Size Limit
In addition to the absolute square footage limit, Chelmsford 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.
Important: State Baseline Data
The information on this page reflects Massachusetts state baseline rules. Chelmsford may have additional local requirements. We recommend confirming with your local building department before starting your project.
Chelmsford ADU Rules at a Glance
Max Size
900 sq ft
% of Home
50%
Parking
1 space(s)
Approval
BY RIGHT
Owner Occupancy
Not required
Rental Allowed
Yes
Short-Term Rental
Not allowed
Chelmsford Building Department
Frequently Asked Questions
What is the maximum ADU size in Chelmsford, MA?
In Chelmsford, 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.
Is ADU size based on my home's size in Chelmsford?
Yes. In Chelmsford, 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 Chelmsford?
Chelmsford 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 Chelmsford?
With 900 sq ft allowed in Chelmsford, 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 Chelmsford?
The information on this page reflects Massachusetts state baseline rules under the Affordable Homes Act. Chelmsford 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 Chelmsford?
Visit our complete ADU rules page for Chelmsford, 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 Chelmsford.