ADU Rules in Boston, Massachusetts

Boston is Massachusetts's capital and largest city, with a population of about 675,000. A dense, walkable city with world-class universities (Harvard, MIT are nearby in Cambridge; Boston University, Northeastern, and others are in Boston), Boston has a severe housing shortage. The city has been actively working to expand ADU allowances, and Massachusetts enacted a statewide ADU law (Chapter 150, Acts of 2024) requiring ADUs by right in single-family zones statewide.

Massachusetts Statewide ADU Law (2024)

Massachusetts Chapter 150, Acts of 2024 requires municipalities — including Boston — to allow accessory dwelling units by right in single-family residential zoning districts. The law limits excessive restrictions on:

• Minimum ADU size (cannot be larger than primary unit or 900 sq ft, whichever is less)
• Parking requirements (limited ability to require ADU parking)
• Owner-occupancy requirements (cannot be required)

Verify how Boston has implemented this state law with the Inspectional Services Department.

Boston Zoning Code

Boston uses a Zoning Code administered by the Boston Planning and Development Agency (BPDA). Boston's zoning can be complex — many parcels have specific Article 80 project review, planned development areas (PDAs), and neighborhood overlay districts.

Permitted ADU Types

Per Boston's ADU policies and state law:

• Basement and interior conversions: Common in Boston's dense rowhouse stock
• Attached ADUs: Additions with separate entrances
• Detached ADUs: In rear yards where lot size permits
• Carriage house / coach house conversions: In appropriate areas

Size Limits

Per Massachusetts ADU law and Boston zoning:

• ADU cannot exceed smaller of: primary dwelling unit size or 900 square feet
• Lot coverage restrictions apply per zone

Setbacks

Per Boston residential zone:

• Rear: Minimum setback (typically 10-20 feet depending on zone)
• Side: Minimum setback

Historic Districts

Boston has several local historic districts with additional review requirements:

• Beacon Hill Historic District
• Back Bay Architectural District
• Bay Village Historic District
• South End Landmark District

Properties in historic districts require review by the Boston Landmarks Commission or the relevant district commission.

Massachusetts Chapter 91 — Waterways

Boston's extensive waterfront areas (Boston Harbor, Charles River, Fort Point Channel) are subject to Massachusetts Chapter 91 (Waterways Act), which protects tidelands and filled tidelands. Properties in Chapter 91 jurisdiction require additional review.

Massachusetts Wetlands Protection Act

Properties within 100 feet of wetlands, 200 feet of rivers, or 100 feet of Boston Harbor are subject to the Massachusetts Wetlands Protection Act. Boston Conservation Commission review is required.

Flood Zones

Coastal Boston has significant FEMA flood zone exposure. Verify flood map status for all waterfront and low-lying properties.

MBTA Communities

Boston is the core MBTA community. The MBTA Communities law (Chapter 40A, Section 3A) requires multifamily housing by right near transit — a related reform to the ADU law.

Permit Process

1. Verify zoning and state ADU law compliance: Boston Inspectional Services
2. BPDA review: May be required for larger projects or in Article 80 areas
3. Historic district review: If in a local historic district
4. Chapter 91 / Conservation Commission: If near waterways or wetlands
5. Check flood zone: If in coastal or low-lying area
6. Submit plans: Boston Inspectional Services Department
7. Building permit and inspections
8. Certificate of Occupancy: Required before occupancy

Contact

Boston Inspectional Services Department 1010 Massachusetts Avenue, Boston, MA 02118 Phone: (617) 635-5300 Website: boston.gov/departments/inspectional-services

Boston Planning and Development Agency (BPDA) One City Hall Square, Boston, MA 02201 Phone: (617) 722-4300 Website: bostonplans.org

Disclaimer: Massachusetts statewide ADU law (2024) has changed local ADU rules — verify current implementation with Boston ISD. Historic districts and Chapter 91 waterfront rules add complexity. Verify all requirements before starting any project.