An accessory dwelling unit (ADU) is a smaller, fully independent home on the same lot as a house, with its own kitchen, bath, and entrance. In Somerville that can mean a detached backyard cottage, an addition to the existing house, or a conversion of a basement, attic, or garage into a legal second unit.
As of February 2, 2025, Massachusetts allows one ADU of up to 900 square feet, or 50% of the principal dwelling’s gross floor area, whichever is smaller, by-right in single-family zones statewide, without a special permit or owner-occupancy requirement. This changed what is possible for Somerville homeowners, though the city still applies its own dimensional and building rules.
Building an ADU in Somerville is as much a zoning and site question as a construction one. Lot size, setbacks, existing coverage, utility connections, and access all shape whether a detached unit, an addition, or an interior conversion makes the most sense on your property.
Detached units: a standalone structure in the yard, subject to setbacks and lot coverage.
Conversions: turning an existing basement, attic, or garage into a code-compliant unit.
Additions: extending the house to create an attached, independent living space.
JRH assesses your lot against both the state ADU law and Somerville’s zoning before design, so the plan is buildable from day one.
The process starts with a free in-home consultation and a feasibility check: we look at your lot, the state by-right ADU allowance, and Somerville’s dimensional rules to confirm what size and type of ADU your property can support. You get an itemized written scope and estimate that includes the zoning and permit path.
Once the design is set, JRH files for the building permit through the City of Somerville’s CitizenServe portal, and licensed electricians and plumbers pull their own permits for the new unit’s systems. Because an ADU is a full dwelling, expect separate utility, plumbing, and electrical work, and possibly a new service connection.
Construction then moves through foundation or demolition, framing or conversion, systems, insulation, inspections, and finishes, closing with a final walkthrough and city sign-off. Timelines depend on whether you are converting existing space or building new.
Yes. Even though the state’s by-right ADU law removed the need for a special permit in single-family zones, an ADU is still a full dwelling that requires a building permit plus separate electrical, plumbing, and gas permits from licensed trades through Somerville’s Inspectional Services Division (ISD), reachable at (617) 625-6600 ext. 5600. The state law caps a by-right ADU at 900 square feet or half the principal dwelling’s floor area, whichever is smaller, and bars a local owner-occupancy mandate, but Somerville still applies setbacks, height, coverage, and building-code requirements, and multi-family or non-conforming lots can involve additional review. JRH confirms your lot’s zoning and the state allowance before design and manages the full permit set through the city’s CitizenServe portal, so your ADU is legal, inspected, and safe to rent or house family in.
Most ADUs in Massachusetts cost between $200,000 and $400,000 to build, at roughly $275 to $450 per square foot, with size driving the range: about $110,000 to $180,000 for 400 square feet, and $275,000 to $450,000 for a full 1,000-square-foot unit (Dwelly, MA, 2026 data). In Somerville, converting an existing basement or garage is usually less expensive than a new detached build, since the structure already exists.
Massachusetts homeowners can offset part of the cost through statewide programs. Mass Save offers rebates for heat pumps, insulation, and other energy-efficient systems in the new unit.
Because JRH is a registered contractor, your ADU is also covered by the state’s Home Improvement Contractor program and Guaranty Fund. We give a free, itemized in-home estimate, with no starting-at guesswork.
JRH builds ADUs throughout Somerville, including Davis Square, Union Square, Ball Square, Teele Square, Winter Hill, Spring Hill, East Somerville, Prospect Hill, and Assembly Square. Somerville is one of the most densely populated cities in the United States, in Middlesex County, covering ZIP codes 02143, 02144, and 02145, where added rental or family units are in high demand near the Green Line Extension.
JRH Constructions is a licensed Massachusetts home improvement contractor (MA HIC #204867), registered and active, fully insured, with every permit pulled and inspected through the City of Somerville. Based in Lynn and working across Somerville’s triple-deckers, Victorians, and condo conversions, JRH pairs local zoning knowledge with code-compliant work. Serving Somerville and Middlesex County. Call (781) 520-1982 for a free in-home estimate.
Common questions Somerville homeowners ask before building an ADU:
Are ADUs allowed by-right in Somerville?
Yes. As of February 2, 2025, Massachusetts allows one ADU of up to 900 sq ft, or half the main home's floor area, by-right in single-family zones with no owner-occupancy requirement. Somerville still applies its own setbacks, height, and building rules.
How much does an ADU cost in Somerville, MA?
Most Massachusetts ADUs run $200,000 to $400,000, about $275 to $450 per sq ft (Dwelly, 2026). A basement or garage conversion is usually cheaper than a new detached unit because the structure already exists.
What types of ADU can I build in Somerville?
A detached backyard unit, an attached addition, or a conversion of a basement, attic, or garage. Which one fits depends on your lot size, setbacks, and existing coverage, which we check before design.
Do I need permits for an ADU even though it's by-right?
Yes. By-right removes the special permit in single-family zones, but an ADU still needs a building permit plus electrical, plumbing, and gas permits from licensed trades through Somerville ISD. JRH manages the full set.
Are you licensed and insured?
Yes. JRH Constructions is a Massachusetts-licensed home improvement contractor (MA HIC #204867), fully insured, and we permit and inspect every project through the City of Somerville.
