Cambridge, MA Accessory Structures: ADU Rules (2026)
ADU rules in Cambridge, MA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- 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
Summary
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.
No zoning ordinance or by-law shall prohibit, unreasonably restrict or require a special permit or other discretionary zoning approval for the use of land or structures for a single accessory dwelling unit, or the rental thereof, in a single-family residential zoning district; provided, that the use of land or structures for such accessory dwelling unit under this paragraph may be subject to reasonable regulations, including, but not limited to, 310 CMR 15.000 et seq., if applicable, site plan review, regulations concerning dimensional setbacks and the bulk and height of structures and may be subject to restrictions and prohibitions on short-term rental, as defined in section 1 of chapter 64G. The use of land or structures for an accessory dwelling unit under this paragraph shall not requi
Full Breakdown
The Massachusetts Affordable Homes Act, signed in August 2024, amended MGL c.40A s.3A to require that every municipality in the state allow at least one accessory dwelling unit by right on every lot where a single-family home is allowed, up to 900 square feet or half the size of the primary unit. Cambridge has implemented this through updates to its zoning ordinance, and because Cambridge already permitted many forms of accessory units in two-family and multi-family districts, ADU approvals are broadly available throughout the city. Owner-occupancy cannot be required under the state law, and short-term rental of an ADU is restricted by the Cambridge STR registration rules. Permits go through the Cambridge Inspectional Services Department and design review under the zoning ordinance. Historic district properties face additional review from the Cambridge Historical Commission.
Frequently Asked Questions
Can I build an ADU in my Cambridge backyard without a special permit?
Can Cambridge require me to live on site if I build an ADU?
Sources & Official References
Other rules in Cambridge
How Cambridge compares: Easiest Cities to Build an ADU·Compare Cambridge to another location·View the Massachusetts accessory structures overview
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ADU Rules in Nearby Cities
How other cities in this county handle adu rules.