ADU Rules in Washington, MA
Washington follows the Massachusetts state baseline with local regulations.
State Baseline Only
This page uses Massachusetts state baseline rules. Local setbacks, height limits, and other dimensional standards may vary. Contact your local building department for specifics.
State Baseline: Affordable Homes Act
Up to 900 sq ft or 50% of principal dwelling (whichever is smaller). By-right approval statewide.
Zoning Districts
RES — Default Residential
Default residential zone (state baseline only — specific district not yet researched)
| Type | Approval | Max Size | Height | Parking | Owner Occ. |
|---|---|---|---|---|---|
| Detached ADU | By Right | 900 sq ft | — | 1 space | No |
| Attached ADU | By Right | 900 sq ft | — | 1 space | No |
| Interior Conversion | By Right | 900 sq ft | — | None | No |
Building Department
Explore ADU Topics in Washington
ADU Zoning in Washington, MA
ADU Permit Process in Washington, MA
ADU Costs in Washington, MA
ADU Size Limits in Washington, MA
ADU Setback Requirements in Washington, MA
ADU Parking Requirements in Washington, MA
ADU Types Explained in Washington, MA
ADU Financing in Washington, MA
ADU Timeline in Washington, MA
ADU Property Value Impact in Washington, MA
ADU Rental Income in Washington, MA
Frequently Asked Questions
Are ADUs allowed by right in Washington, MA?
Yes. ADUs in Washington are approved by right — you submit your plans to the building department and receive a building permit without a public hearing or special permit process. This is guaranteed by the Affordable Homes Act.
What types of ADUs can I build in Washington?
Washington allows detached, attached, conversion ADUs. With up to 900 sq ft allowed, a detached ADU gives you the most flexibility for a full living space.
Do I need to live on the property to build an ADU in Washington?
No. Washington does not require owner occupancy — this is a right guaranteed by the Affordable Homes Act. You can rent both your main home and the ADU, making this a pure investment property if you choose.
How big can I build an ADU in Washington, MA?
In 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.
Does Washington have historic district rules for ADUs?
Yes. Washington has historically significant areas including Brighton Center Historic District, Longfellow House-Washington's Headquarters National Historic Site. If your property is in or near a historic district, your ADU design may require review by the local historical commission for architectural compatibility. This can affect timelines and design choices.
Can I rent out my ADU in Washington?
Yes, long-term rentals are allowed for ADUs in Washington. However, short-term rentals (like Airbnb) are not currently allowed for ADUs.
How accurate is the ADU data for Washington?
The information on this page reflects Massachusetts state baseline rules under the Affordable Homes Act. Washington may have additional local requirements that differ from the state baseline. We recommend confirming with your local building department before starting your project.