ADU Types Explained in Boston, MA
Learn about detached, attached, and conversion ADUs in Boston, Massachusetts. Compare costs, timelines, and which type is right for your property.
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.
ADU Types Available in Boston
Boston allows 3 type(s) of ADU. Each has different cost, timeline, and design implications.
- Detached ADU — a standalone building in your yard with its own entrance, utilities, and foundation. Best for privacy and rental income.
- Attached ADU — an addition connected to your main home that shares at least one wall. Lower construction costs than detached.
- Conversion ADU — transforms existing space like a basement, attic, or garage into a separate dwelling. Fastest and cheapest to build.
Which ADU Type Is Right for You?
The best ADU type for your property in Boston depends on your lot size, budget, and goals. If you want maximum rental income and privacy, a detached ADU is ideal. If budget is your priority, a conversion ADU reuses existing space and avoids foundation costs. An attached ADU offers a middle ground — lower cost than detached with a separate entrance. With 1250 sq ft allowed, you have room for a comfortable 1-2 bedroom unit.
Learn More About ADU Rules in Boston
For full zoning details, see our ADU zoning guide for Boston. To understand what you'll spend, visit our ADU costs page for Boston.
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 are the different types of ADUs I can build in Boston?
Boston allows detached, attached, conversion ADUs. Detached ADUs are standalone structures in your yard. Attached ADUs share a wall with your main home. Conversion ADUs transform existing space like a basement, attic, or garage. Each type has different costs, timelines, and design implications.
Which ADU type is cheapest to build in Boston?
Conversion ADUs are the most affordable option in Boston at $100-$200/sq ft, since you reuse existing structure and avoid foundation costs. See our ADU costs page for Boston for a full breakdown.
Can I build a detached ADU in my backyard in Boston?
Yes, detached ADUs are allowed in Boston. Your ADU must meet setback requirements — typically 5 ft from the rear property line. Check our ADU zoning page for Boston for exact setback distances.
Can I convert my garage into an ADU in Boston?
Yes, garage conversions are allowed in Boston. Converting an existing garage is one of the fastest and most affordable ways to add an ADU — you're reusing the existing structure and foundation.
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.