Everything homeowners need to know about building an accessory dwelling unit in Somerville - size limits, setbacks, permits, and how the city's own head start on ADUs lines up with the 2024 state law that now protects them everywhere in Massachusetts.
Last verified: September 2026
9/10 · Very ADU-friendly
900 sq ft*
Max ADU size
12 ft*
Max height
Not required
Owner-occupancy
By-right
Permit type
This page summarizes the Somerville Zoning Ordinance's accessory dwelling unit standards alongside the 2024 Massachusetts Affordable Homes Act for general reference - it is not legal advice. Regulations change; always confirm current requirements with Somerville's Office of Strategic Planning & Community Development (OSPCD) and Inspectional Services Division (ISD) before designing or applying for permits. See sources below.
Zoning & size requirements
What you're allowed to build
ADU type
Max size
Max height
Notes
Detached (Backyard Cottage)
576 sq ft floor plate
1.5 stories / 12 ft
Somerville's own accessory-building type, defined separately from the state's general ADU formula
Attached, internal, or a larger detached unit ("Protected Use ADU")
Up to 900 sq ft or 50% of the primary dwelling's gross floor area, whichever is smaller
Attached: same as primary structure. Detached: confirm with OSPCD
The broader as-of-right floor guaranteed statewide since February 2025
Allowed zoning districts
By right in Somerville's Neighborhood Residence (NR) districts, and citywide wherever the underlying use is a single- or two-family dwelling, consistent with the 2024 state mandate. Somerville's dense mix of single-family, two-family, and triple-decker lots means eligibility and exact dimensional limits vary block by block - confirm your parcel's zoning and which ADU pathway fits your project with OSPCD.
Additional standards
Setbacks, parking & other rules
Setbacks
No ADU-specific setback beyond the standard accessory-structure setbacks for your zoning district - confirm exact dimensions with OSPCD
Parking
No mandatory off-street parking in most of the city; Somerville eliminated most citywide parking minimums as part of its 2019-2022 zoning changes
Units per lot
1 ADU permitted by right in addition to the primary dwelling; a second ADU on the same lot requires a special permit under the state framework
Owner-occupancy
Not required, either under Somerville's own by-right ordinance or the 2024 state law
Building code
Must meet Massachusetts State Building Code dwelling-unit standards
Affordable-unit trigger
If your ADU is the 3rd dwelling unit created on a Neighborhood Residence lot, that unit generally must be permanently affordable middle-income housing
Historic districts
Exterior changes within one of Somerville's local historic districts go through OSPCD Historic Preservation review in addition to standard permitting
Ordinance history
How Somerville's ADU rules got here
Pre-2019ADUs not allowed anywhere in the city. Backyard cottages and other accessory dwelling units had no path to approval under Somerville's old zoning code.
2019First zoning overhaul in 30 years. The City Council's December 2019 rewrite legalized backyard cottages and other ADUs in Neighborhood Residence districts, but required a discretionary site plan review, including two neighborhood meetings and a hearing before the Urban Design Commission.
2022ADUs made by right. In September 2022, the City Council voted unanimously to drop the discretionary review, allowing backyard cottages and other ADUs to be built by right anywhere in a Neighborhood Residence district.
2025State law reinforces by-right status statewide. The Massachusetts Affordable Homes Act's ADU protections took effect February 2, 2025, guaranteeing a by-right pathway up to 900 sq ft (or 50% of the primary home) and barring any owner-occupancy requirement citywide.
Worth knowing: Somerville had been moving this direction for years on its own - eliminating parking minimums and legalizing backyard cottages well before the state stepped in. Statewide, Massachusetts reported 1,224 ADUs permitted or approved across 217 communities in the first year after the 2024 law took effect, a reminder that even permissive zoning translates into construction gradually.
Permit process
How Somerville ADU permitting works
1
Confirm your zoning district & ADU type
Check whether your project fits Somerville's by-right Backyard Cottage rules or the broader state "Protected Use ADU" floor, and confirm eligibility with OSPCD or ISD before you design.
2
Prepare your plans
You'll need a to-scale site plan and floor plans/elevations showing the ADU's size, entrance, and its relationship to the primary structure and property lines.
3
Apply through CitizenServe
Submit your building permit application online through the city's CitizenServe portal at somervillema.gov/citizenserve.
4
Plan review
ISD reviews your application for zoning and Massachusetts State Building Code compliance; expect a follow-up request if anything's missing.
5
Building permit & inspections
Once approved, pull your building permit, begin construction, and schedule the standard framing, plumbing, electrical, and final inspections.
6
Certificate of occupancy
A final inspection and sign-off is required before the ADU can be occupied or rented.
Fees and timelines vary by project scope and current department workload - confirm current figures with OSPCD or ISD before you budget or schedule your project.
Budgeting
What it costs to build an ADU in Somerville
Garage conversion
Roughly $85,000-$180,000, the least expensive path since the shell already exists
Basement conversion
Roughly $115,000-$225,000, depending on existing ceiling height and utility access
New detached ADU (Backyard Cottage)
Roughly $160,000-$330,000+ for a code-compliant, fully insulated freestanding unit
Building permit fee
Calculated from estimated construction cost per Somerville's standard fee schedule - confirm current rates with ISD
These are general ranges from local contractor estimates and regional reporting, not quotes - actual cost depends heavily on lot conditions, finishes, and whether utilities need to be extended. Somerville's tight urban lots and older housing stock often make small-lot logistics, not zoning, the bigger cost driver.
Before you apply
Tips for a smooth approval
Confirm which ADU pathway applies to your project. Somerville's own Backyard Cottage rules and the state's broader 900 sq ft Protected Use ADU floor aren't identical - OSPCD can tell you which one fits your plan.
Ask about the affordability trigger before you design a third unit. If your ADU would be the third unit on a Neighborhood Residence lot, it may need to be a permanently affordable unit - worth knowing before you finalize floor plans.
Don't over-design for parking you may not need. Somerville dropped most citywide parking minimums, so confirm current requirements before you sacrifice yard space to a parking pad.
Check whether you're in a local historic district early. Exterior changes there go through an additional Historic Preservation review, which can add real time to visible additions.
Budget for a tight urban lot, not just the structure. Small lot sizes and shared side yards are the more common constraint in Somerville, more so than the zoning caps themselves.
FAQ
Common questions
Do I need to live on the property to build an ADU in Somerville?
No. Owner-occupancy is not required, either under Somerville's own by-right ordinance or under the 2024 Massachusetts Affordable Homes Act.
How many ADUs can I build on one lot?
One, in addition to the primary dwelling. A second ADU on the same lot would require a special permit under the state's framework.
Do I need extra parking for an ADU?
No. Somerville eliminated most citywide off-street parking minimums as part of its 2019-2022 zoning overhaul.
What's the maximum size for an ADU in Somerville?
It depends on the type. A detached Backyard Cottage is capped at 576 sq ft (1.5 stories / 12 ft). Other configurations can use the state's broader floor of up to 900 sq ft or 50% of the primary home's floor area, whichever is smaller. Confirm with OSPCD which one applies to your project.
Was Somerville already allowing ADUs before the 2024 state law?
Yes. Somerville legalized backyard cottages in December 2019 (with a discretionary review process) and simplified that to a by-right process in September 2022, well before the statewide mandate took effect in February 2025.
How much does an ADU cost to build in Somerville?
Garage conversions tend to run roughly $85,000-$180,000, basement conversions $115,000-$225,000, and new detached Backyard Cottages $160,000-$330,000 or more, based on local contractor estimates and regional reporting - not counting permit fees.
Do I need to make my ADU an affordable unit?
Only in a specific case: if the ADU is the third dwelling unit created on a lot in a Neighborhood Residence district, that third unit generally must be permanently affordable middle-income housing. Confirm applicability with OSPCD.
How long does the permit process take?
Timelines vary with project scope and current department workload - confirm current estimates with OSPCD or ISD before you finalize a construction schedule.
Can I rent my ADU out on Airbnb or a short-term rental platform?
Somerville regulates short-term rentals separately from standard housing rules. If that's the plan, confirm current short-term rental requirements with the city before you finalize a design that depends on that income.
