Accessory Structures in Cambridge, MA: What Residents Actually Need to Know
If you live in Cambridge or are thinking about moving there, accessory structures are one of those things you probably won't think about until they affect you directly. Cambridge has 5 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.
Carport Rules
Carports in Cambridge are accessory structures subject to zoning setbacks and a building permit over 120 sq ft. They are uncommon on dense lots and historic districts often restrict open-wall designs.
Key details: Building permit threshold: Over 120 sq ft. Setbacks: 3 to 5 feet typical. Historic district review: Often restrictive. Front yard impact: May require variance. Electrical: Separate permit.
Contact your local code enforcement office for specific penalty information.
Tiny Homes
Tiny homes on foundations are allowed as ADUs in Cambridge under MGL c.40A s.3A. Tiny homes on wheels are treated as recreational vehicles and cannot be used as permanent dwellings under the MA State Building Code.
Key details: Tiny home on foundation: Allowed as ADU. Max ADU size: 900 sq ft. Tiny home on wheels: Not allowed as dwelling. Governing code: 780 CMR and c.40A s.3A. Park model units: Not permitted in Cambridge.
Contact your local code enforcement office for specific penalty information.
Garage Conversions
Garage conversions to living space are permitted in Cambridge with full building permits. Under the 2024 MA Affordable Homes Act, converting a garage into an ADU is allowed by right in most zones.
Key details: Full building permit: Required. ADU conversion: By right under state law. Energy code: MA Specialized Stretch Code. Parking replacement: Typically waived for ADU. Historic districts: Extra review.
Contact your local code enforcement office for specific penalty information.
Shed Rules
Cambridge requires a building permit for any shed over 120 sq ft and zoning compliance for smaller sheds. Setbacks of 3 to 5 feet apply, with extra review in historic districts.
Key details: Building permit threshold: Over 120 sq ft. Typical setback: 3 to 5 feet. Max shed height: Generally 15 feet. Historic districts: Commission review required. Permits contact: 617-349-6100.
Contact your local code enforcement office for specific penalty information.
ADU Rules
Cambridge allows accessory dwelling units by right under the 2024 MA Affordable Homes Act and MGL c.40A s.3A, the statewide ADU mandate. Most residential districts now permit one ADU by right.
Key details: State law: MGL c.40A s.3A Affordable Homes Act. By-right status: Yes on single-family lots. Max size: 900 sq ft or half primary unit. Owner occupancy: Cannot be required. Historic districts: Extra commission review.
Contact your local code enforcement office for specific penalty information.
The rules around adu rules in Cambridge lean permissive, but that does not mean anything goes.
The Bottom Line
Cambridge'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 Cambridge is broadly strict or permissive.
This guide is based on Cambridge's current municipal code. Local rules can and do change, so check the individual ordinance pages for the latest details, penalties, and FAQs.