ADU adoption in Waltham off to a slow start

Waltham residents considering building an accessory dwelling unit on their property join a modest but growing number of people taking advantage of new state laws. 

These units can be built by homeowners, often as new freestanding buildings or through renovations to existing structures such as a garage. ADUs can provide housing for family members or be rented for income to homeowners.

Permitting of accessory dwelling units is off to a slow start following the 2024 passage of a state law designed to encourage their construction, according to the Pioneer Institute.

In the first half of 2025, building officials in Massachusetts issued 550 permits for ADUs. By comparison, California has issued 30,000 permits in 2024. On a per capita basis, the rate of permitting in Massachusetts is less than a quarter of California’s rate.

In Waltham, only five permits were issued in 2025, according to Chief Building Inspector Brian Bower.

Bower encourages individuals interested in an ADU to meet with him in his office, located in the government center on School Street. He said he will explain the process for obtaining a permit and help residents assess whether their property meets the city’s requirements.

The city’s ADU ordinance, adopted in 2025, details the general requirements for an ADU, which “shall include a living area, kitchen, bathroom, and sleeping area separate from the principal dwelling unit.”

A number of other requirements must be met as well. ADUs are permitted only as an accessory to a principal dwelling unit within single-family residential zoning districts. Zoning and other information is available on the city’s online map resource.

The minimum floor area of an ADU is 450 square feet and at least one parking place must be provided unless the lot is located within half a mile of a transit station or bus stop. An ADU must be exclusively for residential use and may not be used for short-term rentals. 

Bower noted that some plans for basement conversions run into difficulty due to the need for a separate entrance, though that can be through a common entry hall if the state requirements for safe egress are met.

The ordinance stipulates additional conditions, and a trip to the city Building Department is the best way for residents to ensure that an ADU is feasible on their property, Bower emphasizes. It’s an important step before developing detailed construction plans required to obtain a building permit.