Building an ADU or backyard cottage in Somerville

What you can build in Somerville

Somerville's zoning ordinance uses "building types" instead of a generic ADU rule. In the Neighborhood Residence (NR) district, "one (1) principal building and one (1) accessory building are permitted on each lot." The principal building types allowed by right are the Cottage, Detached House, Semi-Detached House, Duplex and Detached Triple Decker. The accessory building type allowed by right is the Backyard Cottage.

The ordinance describes a Backyard Cottage as "a small floor plate, detached, accessory building type typically providing space for one (1) small dwelling unit, a home occupation, a playhouse for children, or vehicular parking on the same lot as a principal building type." The NR district's stated purpose includes creating "dwelling unit types, sizes, and bedroom counts ideal for smaller households in Cottages and Backyard Cottages."

You can also add units inside the main house. A Detached House may have one to three vertically stacked units. Check your lot's district on the city's Zoning Atlas. See our Massachusetts ADU guide and what is an ADU. [1][4]

Backyard cottage size, height and setbacks

The Backyard Cottage standards in Section 3.1.12 set:

• Units: one dwelling unit at most.
• Size: main massing up to 24 feet wide and 32 feet deep, with a floor plate of no more than 576 sq ft.
• Height: up to 1.5 stories, with a story height of up to 12 feet; flat or gable roof.
• Setbacks: at least 3 feet from side and rear lot lines (on an alley, 3 or 20 feet), at least 60 feet from the primary front lot line and 10 feet from a secondary front lot line.
• Separation: at least 10 feet from other buildings.
• Lot width: at least 32 feet, or 34 feet with front driveway access.
• Lot coverage and green score: counted with the principal building.

This section was last amended by Ordinance 2023-23, effective November 21, 2023. Check the Somerville Zoning Ordinance for any later change before you design. [1]

Parking

Somerville does not require off-street parking for a backyard cottage in NR. The district's parking table (Table 3.1.18, as amended by Ordinance 2024-15) lists no motor vehicle parking requirement for residential uses, and the ordinance says "there are no parking requirements for accessory uses."

If you add a driveway, it needs a permit from the City Engineer, may be up to 12 feet wide, and is not allowed in the frontage area between a building and the front lot line. [1]

Design and safety requirements

A Backyard Cottage must have:

• a paved walkway at least 3 feet wide that can take an ambulance cot to and from the cottage;
• utility services buried underground;
• an automatic sprinkler system, by order of the Fire Prevention Bureau, if it is used as a residence, unless the lot is a corner lot, has a driveway leading to the cottage, or abuts an alley.

[1]

How state ADU law applies

Somerville's zoning ordinance says it is enacted under M.G.L. Chapter 40A and "does not restrict the use of land or structures for activities protected by M.G.L. Chapter 40A, Section 3." So the state's protected-use ADU rules, in effect since February 2, 2025, set a floor here. Under M.G.L. c. 40A, Section 3, a city may not "prohibit, unreasonably restrict or require a special permit" for a single ADU in a single-family residential zoning district. The state regulation defines that as any district where single-family homes are allowed, which includes NR.

For a protected-use ADU, the city cannot require that you or anyone else live in the ADU or the main home. It can require at most one extra parking space, and none if any part of your lot is within half a mile of a subway, commuter rail, ferry or bus station. The state caps a protected ADU at half the main home's gross floor area or 900 sq ft, whichever is smaller.

If Somerville's building-type standards and state law seem to conflict for your lot, state law generally controls for a single protected ADU. The state housing office says it cannot answer questions about specific properties, so confirm with the city's ISD zoning team. [1][2][3][5][6]

Converting a garage or existing outbuilding

A garage that doesn't meet the Backyard Cottage standards (for example, one closer than 60 feet to the front lot line) may not fit the building type as drawn. The state regulation says a city "may not prohibit the development of a Protected Use ADU in an existing structure" because of nonconformity, if it can be converted in line with the building code and state law. Ask ISD how this applies to your structure. Our garage conversion guide covers the general steps. [1][3]

Permits

Any new construction or alteration needs a building permit from the Inspectional Services Department (ISD) before work starts. The city says a building permit "signifies that the proposed use or structure is meeting zoning prerequisites (e.g.; setback, frontage, or minimum yard-size requirements) and the State Building Code."

Apply online through CitizenServe: create an account, apply for a permit and choose "building permit." Homeowners of a 1- or 2-family home who live there may pull the permit themselves; they submit proof of residence, an Estimated Cost Affidavit, plans if needed, a Workplace Safety Affidavit and a Wage Theft Affidavit. For a 3-family or larger property, a licensed contractor must apply. Licensed electricians and plumbers pull their own permits.

Some projects need Site Plan Approval, a Special Permit or a Hardship Variance; in NR the Zoning Board of Appeals decides those. ISD is at 1 Franey Rd.; call 617-625-6600 ext. 5600 or email isd@somervillema.gov. See our ADU permit process guide and ADU cost guide. [1][7][8]

Permit steps in Somerville

1. Confirm your zoning district

   Look up your lot in the Zoning Atlas. Backyard Cottages are allowed by right in NR. Questions go to isdzoning@somervillema.gov. [1][4]

2. Design to the Backyard Cottage standards

   One unit, a floor plate of up to 576 sq ft, 1.5 stories, 3-foot side and rear setbacks, 60 feet back from the front lot line, a 3-foot paved walkway and buried utilities. [1]

3. Check sprinkler needs

   Plan a sprinkler system unless the lot is a corner lot, has a driveway to the cottage, or abuts an alley. [1]

4. Apply for a building permit in CitizenServe

   Create an account at somervillema.gov/citizenserve, choose building permit, and upload the required affidavits and plans. [7]

5. Get zoning relief only if needed

   If the design needs Site Plan Approval, a Special Permit or a Hardship Variance, the Zoning Board of Appeals decides it. [1]

6. Build and pass inspections

   Display the permit at the job site. Construction hours are Monday to Friday 7 a.m. to 7 p.m. and Saturday 9 a.m. to 7 p.m. [7]

Who to call

• Inspectional Services Department (ISD), Building Division

  Building permits, plan review, inspections

  (617) 625-6600 x 5600

  1 Franey Rd., Somerville, MA 02145

  [7][8]

• ISD Zoning team

  Interpreting the Zoning Ordinance and Zoning Atlas

  [4]

Questions people ask