Accessory Structures in Boston, MA

If you live in Boston or are thinking about moving there, accessory structures are one of those things you probably won't think about until they affect you directly. Boston has 4 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

Shed & Carport Rules

A shed or other accessory building in Boston's rear yard is capped at 15 feet in height and must sit at least 4 feet from any side lot line under Zoning Code Article 20, Section 20-2. In single-family, two-family and rowhouse districts, all accessory buildings in one rear yard together cannot exceed the coverage percentage Table B sets for that district.

Key details: Max accessory building height: 15 ft. Min side lot line setback: 4 ft. Rear yard coverage cap: Set by Table B (e.g., 25% in S-.3). Coverage cap applies in: S, R and H districts. Clearance from other yard projections: 8 ft from accessory building.

The Building Commissioner enforces Article 20 under Section 5-1. An accessory building taller than 15 feet, built closer than 4 feet to a side lot line, or exceeding the Table B rear-yard coverage cap for its district is a zoning violation subject to a removal or correction order, and erecting one without the required building permit compounds the violation.

ADU Owner Occupancy

The Zoning Code allows one detached and one non-detached accessory dwelling unit per lot, but conditions both on occupancy: Article 2's definitions require the property owner to be the principal resident for a non-detached ADU and a resident for a detached ADU, on top of separate size, height and rental-registry conditions.

Key details: Non-detached ADU size cap: 75% of principal unit or 1,250 sf, smaller controls. Detached ADU floor plate cap: 900 sf or primary structure's plate, smaller controls. Detached ADU height cap: 1½ stories or primary structure's height. Detached ADU setback: 5 feet from any other structure. Occupancy condition: Owner must be resident / principal resident.

An ADU built without meeting the owner-occupancy condition in Article 2's definition does not qualify as an allowed accessory use; Article 4, Section 4-1 makes any structure used out of conformity with its district's regulations unlawful, and Section 4-3 bars a use permit until the Building Commissioner certifies compliance. The Building Commissioner enforces the code under Section 5-1, and an owner who no longer occupies the property loses the basis for the unit's accessory-use status.

ADU Rules

ADUs are now allowed by right statewide in Massachusetts under the 2024 Affordable Homes Act, with a maximum of 900 square feet. Boston cannot require owner-occupancy or special permits for protected ADUs.

Key details: Max Size: 900 sq ft or 50% of home. Approval: By right statewide. Owner-Occupancy: Cannot be required. Transit Parking: None within 0.5 mi.

Building permits are still required for construction. Non-compliant ADUs face standard zoning and building code enforcement. Unpermitted construction subject to fines and required remediation.

Garage Conversions

Garage conversions to ADUs are explicitly allowed under the 2024 MA Affordable Homes Act by right on single-family zoned lots, subject to building code compliance and permit requirements.

Key details: ADU Type: Allowed by right (2024 law). Permit: Building permit required. Max Size: 900 sq ft / 50% of home. Code: 780 CMR full compliance.

Unpermitted conversions subject to ISD enforcement including fines and required remediation. Uninspected conversions may not receive certificate of occupancy. Insurance and sale complications.

The Bottom Line

Boston'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 Boston is broadly strict or permissive.

These rules come from Boston's publicly available municipal code. For complete penalty schedules, exemption details, and answers to common questions, see the individual ordinance pages throughout this guide.