ADU rules in Waltham, MA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Waltham must permit Accessory Dwelling Units by-right under the 2024 Massachusetts Affordable Homes Act (MGL c.40A s.3A). ADUs up to 900 sq ft are allowed on single-family lots without special permits.
Under the Massachusetts Affordable Homes Act signed in August 2024, all cities and towns including Waltham must permit Accessory Dwelling Units (ADUs) by-right on any lot zoned for single-family use. The state law (MGL c.40A s.3A as amended) took effect February 2, 2025, and preempts local zoning that would prohibit or unreasonably restrict ADUs. Key provisions: ADUs up to 900 square feet or 50 percent of the main dwelling (whichever is smaller) must be allowed; no owner-occupancy requirement may be imposed; no additional parking requirement beyond one space; and short-term rentals of ADUs (under 30 days) can be prohibited by the city. Waltham has updated its zoning ordinance to comply and requires a standard building permit rather than a special permit. Detached, attached, and interior ADUs (basement, attic conversions) are all permitted. The Waltham Building and Zoning offices handle applications.
Contact your local code enforcement office for specific penalty information.
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Side-by-side rule comparisons with other cities in Middlesex County.
See how other cities in Middlesex County handle adu rules.
See how Waltham's adu rules rules stack up against other locations.
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