Accessory Dwelling Units (ADUs) in Somerville, MA

Written By Tyler Munroe

Somerville has been proactive in embracing Accessory Dwelling Units (ADUs) as a means to address housing needs and promote diverse living arrangements. With the introduction of the Affordable Homes Act, homeowners in Somerville have new opportunities to add ADUs to their properties.

Here's what you need to know about building an ADU in Somerville as of 2025.

Statewide ADU Regulations: The Affordable Homes Act

In August 2024, Massachusetts enacted the Affordable Homes Act, a significant piece of legislation aimed at addressing the state's housing shortage. A key provision of this act is the allowance of ADUs by right in single-family zoning districts.

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 regulations aim to streamline the process for homeowners and encourage the development of diverse housing options across the state.

Somerville's Local ADU Policies

Prior to the Affordable Homes Act, Somerville had already taken steps to facilitate the creation of ADUs. In 2019, the city overhauled its zoning code, allowing for backyard cottages and other forms of ADUs in the Neighborhood Residence district. Initially, these units required a site plan approval process, including neighborhood meetings and a hearing before the Urban Design Commission. However, in September 2022, the City Council voted to simplify this process, making it easier for property owners to convert backyard structures into rental apartments or condos.

Despite these advancements, certain challenges remain. For instance, in Neighborhood Residential zones, properties are limited to three units, one of which must be permanently affordable. This requirement can impact the feasibility of adding an ADU, especially for properties that already have multiple units.

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

  • Owner Occupancy: While the Affordable Homes Act prohibits municipalities from requiring owner occupancy for ADUs, Somerville's previous regulations included such requirements. The city is expected to align its policies with the state law, removing owner-occupancy mandates.

  • Permitting Process: The state law allows for by-right construction of ADUs, eliminating the need for special permits. Somerville has taken steps in this direction, but certain projects may still require additional approvals, depending on the property's zoning and existing structures.

  • Parking Requirements: Under the Affordable Homes Act, no additional parking is required for ADUs within 0.5 miles of public transit. Somerville's zoning code has been progressive in reducing parking requirements, especially in transit-accessible areas, aligning with the state's approach.

What This Means for Homeowners in Somerville

If you're a homeowner in Somerville considering adding an ADU to your property, here's what you should know:

  • Permitting: With the Affordable Homes Act, you can build an ADU by right, eliminating the need for special permits, provided your project meets the stipulated criteria.

  • Design considerations: Ensure your ADU design complies with both state and any applicable local regulations, especially concerning size and setbacks.

  • Consult local authorities: While state law provides overarching guidelines, it's crucial to consult Somerville's local planning or zoning department for any additional requirements or recommendations.

Get a Custom ADU Roadmap for Your Property

Considering building an ADU in Somerville? 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 Somerville's ADU policies, please refer to the official city documents or contact the local planning department directly.

Tyler Munroe
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