Accessory Dwelling Units (ADUs) in Boston, MA

Written By Tyler Munroe

Boston has been proactive in exploring Accessory Dwelling Units (ADUs) as a solution to its housing challenges. However, the city's current regulations differ from the broader provisions outlined in Massachusetts' Affordable Homes Act. Here's what homeowners need to know about building an ADU in Boston as of 2025.​

Boston's Current ADU Policies

Boston permits homeowners of 1-, 2-, and 3-family homes to build ADUs on the property where they reside. These units can be internal (e.g., basement or attic conversions) or, in some cases, external additions. However, the city requires that the homeowner lives on the same parcel where the ADU is constructed. Additionally, ADUs must comply with specific design and zoning regulations, and obtaining the necessary permits involves a multi-step process, including design approval and adherence to building codes. Boston.gov​

Massachusetts' Affordable Homes Act: A Broader Approach

In contrast, the state's Affordable Homes Act, effective February 2025, mandates that municipalities allow one ADU by right on residential properties with up to four units. Key provisions include:​

  • By-right ADUs: Homeowners can build ADUs without needing special permits, provided they meet certain criteria.​

  • Size limitations: ADUs must be no larger than 900 square feet or 50% of the primary dwelling's gross floor area, whichever is smaller.​

  • No owner-occupancy requirement: Municipalities cannot mandate that either the main house or the ADU be owner-occupied.​

  • Parking requirements: No additional parking is required if the property is within 0.5 miles of public transit. ​

These statewide regulations aim to streamline the process for homeowners and encourage the development of diverse housing options across Massachusetts.​

Key Differences Between Boston's Policies and the Affordable Homes Act

  • Owner Occupancy: Boston requires homeowners to live on the property where the ADU is built, whereas the state law prohibits such a requirement.​

  • Permitting Process: Boston's process involves multiple steps, including design approval and adherence to local zoning codes. The state law simplifies this by allowing ADUs by right, eliminating the need for special permits.​

  • Parking Requirements: Boston may have specific parking requirements based on the property's location, while the state law removes additional parking mandates for properties near public transit.​

What This Means for Boston Homeowners

While the Affordable Homes Act sets a new standard for ADU development in Massachusetts, Boston's current policies have not fully aligned with the state law. Homeowners in Boston should be aware of the existing local regulations and the potential for future changes as the city adapts to the statewide mandates.​

Get a Custom ADU Roadmap for Your Property

Considering building an ADU in Boston? We offer a comprehensive ADU Roadmap Report tailored to your property. This includes:

  • Site visit and feasibility analysis

  • Cost estimation and timeline

  • Guidance on permitting processes

  • Rental income projections

  • Contractor referrals

👉 Schedule Your ADU Roadmap Consultation

Note: For the most accurate and up-to-date information on Boston's ADU policies, please refer to the official city documents or contact the local planning department directly.​

Tyler Munroe
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