ADU Rules: Arlington vs Cambridge
How do adu rules rules compare between Arlington, MA and Cambridge, MA?
Arlington and Cambridge have similar restriction levels.
Arlington, MA
Middlesex County
Under the 2024 MA Affordable Homes Act (MGL c.40A s.3A), ADUs up to 900 sq ft are allowed by-right on every single-family lot in Middlesex County. No owner-occupancy or special permit required.
View full Arlington rules →Cambridge, MA
Middlesex County
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.
View full Cambridge rules →Key Facts Comparison
| Fact | Arlington | Cambridge |
|---|---|---|
| State Law | MGL c.40A s.3A (Feb 2025) | - |
| Max Size | 900 sq ft or 50 percent | - |
| Owner-Occupancy | Not required | - |
| Parking Max | 2 spaces max | - |
| Review Type | By-right, no special permit | - |
| 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 |
Highlighted rows indicate differences between cities.
Arlington FAQ
Can I build an ADU on my lot in Middlesex County?
Yes. As of February 2025, the Affordable Homes Act requires all MA municipalities to allow ADUs by right on single-family lots up to 900 sq ft.
Do I have to live on the property?
No. The statewide ADU law prohibits owner-occupancy requirements, so you can rent both units.
Cambridge FAQ
Can I build an ADU in my Cambridge backyard without a special permit?
Yes in most cases. Under the 2024 state law it is by-right up to 900 sq ft, subject to zoning review.
Can Cambridge require me to live on site if I build an ADU?
No. The state law prohibits owner-occupancy mandates for ADUs.
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