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 2 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

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.

The rules around adu rules in Cambridge lean permissive, but that does not mean anything goes.

Shed & Carport Rules

In Cambridge's Residence C-1 zoning district, an accessory building such as a shed or garage cannot sit closer to any lot line than the district's minimum setback, and in Residence C-1 and Office-1 districts it cannot rise above 15 feet, per Zoning Ordinance Sections 4.21.h and 4.21.k.

Key details: Governing sections: Zoning Ordinance Sec. 4.21.h and Sec. 4.21.k. Height cap: 15 feet (Residence C-1 and Office-1). Setback rule: Same as district's minimum principal-building setback. Same-lot rule: Must sit on same lot as principal use. Max fine: $300 per offense, each day separate.

A shed or accessory building built closer to a lot line than the Residence C-1 setback, or taller than 15 feet in a Residence C-1 or Office-1 district, violates Section 4.21. The Superintendent of Buildings can refuse a building permit or certificate of occupancy under Section 9.12, and Section 9.16 authorizes a fine of up to $300 per offense, with each day of noncompliance a separate violation.

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.