ADU Size Limits in Mount Washington, MA
The maximum ADU size in Mount Washington is 900 sq ft, or 50% of your home.
Find the maximum ADU size allowed in Mount Washington, Massachusetts. Includes square footage limits, height restrictions, lot coverage, and bedroom count.
ADU Regulations in Mount Washington
Mount Washington 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. Mount Washington may also have additional local regulations that supplement the state baseline.
Size Limits: Room for a Full Cottage
At 900 sq ft, Mount Washington 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, Mount Washington 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. Mount Washington may have additional local requirements. We recommend confirming with your local building department before starting your project.
Mount Washington 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
Mount Washington Building Department
Frequently Asked Questions
What is the maximum ADU size in Mount Washington, MA?
In Mount Washington, 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 Mount Washington?
Yes. In Mount Washington, 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 Mount Washington?
Mount Washington 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 Mount Washington?
With 900 sq ft allowed in Mount Washington, 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 Mount Washington?
The information on this page reflects Massachusetts state baseline rules under the Affordable Homes Act. Mount Washington 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 Mount Washington?
Visit our complete ADU rules page for Mount Washington, 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 Mount Washington.