Somerville's Accessory Structures: The Rules That Matter
Every city handles accessory structures a little differently. In Somerville, Massachusetts, there are 4 distinct rules that residents and property owners should be aware of. Some are stricter than what neighboring cities enforce, and others are more relaxed. Here is what you need to know.
Garage Conversions
Converting a garage to living space in Somerville requires a building permit and may qualify as an ADU under the 2024 Affordable Homes Act with relaxed zoning review.
Key details: Building permit: Required. Code compliance: 780 CMR full residential. As ADU: Up to 900 sq ft by right. Parking loss: Generally not a barrier. Reviewer: Inspectional Services.
ADU Rules
Somerville was an early Massachusetts adopter of ADU zoning and now complies with the 2024 Affordable Homes Act mandating ADUs by right in all single-family zones statewide.
Key details: State mandate: Affordable Homes Act 2024. Max size: 900 sq ft by right. Owner occupancy: Not required. Parking mandate: Prohibited by state. Permit: Building permit required.
Somerville is more permissive than most cities when it comes to adu rules. That said, there are still limits.
Shed & Carport Rules
Somerville caps backyard sheds at 12 feet in height and lets them sit as close as 3 feet from a side or rear lot line, but bars any encroachment into the required front setback. Sheds are permitted by-right, so no special permit is needed if the shed goes up after the main house.
Key details: Max shed height: 12 feet (§10.2.2(u)). Side/rear setback: 3 feet minimum from lot line. Front setback: No encroachment permitted. Special permit needed: No, sheds are by-right. Daily fine for violation: $300 per offense per day.
Somerville enforces zoning noncompliance, including an oversized or misplaced shed, under Section 1.1.7 Fines and Penalties: the Building Official may fine the property owner $300 for each offense, and every day the shed remains out of compliance counts as a separate offense. The Building Official can also pursue enforcement through a non-criminal disposition process under state law instead of stacking daily fines.
Tiny Homes
Somerville's zoning ordinance permits one detached Backyard Cottage per lot in the Neighborhood Residence district, capping the accessory building at a 576-square-foot floor plate, 24 feet wide, 32 feet deep, and 1.5 stories under Zoning Ordinance Section 3.1.12. The cottage holds only one dwelling unit and must sit at least 3 feet from the side and rear lot lines.
Key details: Max floor plate: 576 sq ft. Max stories: 1.5 stories. Max story height: 12 ft. Min side/rear setback: 3 ft. Dwelling units allowed: 1 per cottage.
A Backyard Cottage built beyond the 576-square-foot floor plate, taller than 1.5 stories or 12 feet per story, or sited closer than 3 feet to a side or rear lot line violates Section 3.1.12. The Somerville Inspectional Services Department can stop the work, require removal of the noncompliant structure, or withhold occupancy until the sprinkler and walkway standards are met.
The Bottom Line
Somerville's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Somerville is broadly strict or permissive.
This guide is based on Somerville's current municipal code. Local rules can and do change, so check the individual ordinance pages for the latest details, penalties, and FAQs.