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The key to more housing is in our own backyards

Accessory dwelling units can address housing needs without disrupting the character of communities.

Mary J. Casey, 90, lives in an accessory dwelling unit attached to her son’s home in Abington in 2022. She stands with her daughter, Mary M. Casey, near the overhang that connects the unit, on right, to her son’s home, on left.John Tlumacki/Globe Staff

In Greater Boston, 50 percent of renters are “rent burdened,” spending more than 30 percent of their income on rent, while homeownership opportunities continue to be pushed further out of reach.

Increasing the stock of residences is one of the strongest means of creating more affordable, equitable, and accessible housing in Massachusetts. The state has made great progress in laying the groundwork to allow new multifamily housing by right via the MBTA Communities law, which requires towns and cities served by the MBTA to draft new zoning to spur transit-oriented development. But the state can do more, and one of the solutions is in residents’ own backyards.


Accessory dwelling units are small homes or apartment units that exist on the property of a single-family home. They can stand alone or be attached to the primary dwelling and provide housing for family members, caregivers, or renters. Some cities and towns in Massachusetts have passed zoning laws allowing ADUs by right, reducing the obstacles of obtaining a permit to build a unit.

California passed legislation allowing ADUs by right statewide, so long as certain requirements were met. Since doing so, ADU interest has skyrocketed. In 2021, there were more than 21,000 permits submitted for ADUs in California, up from over 7,000 in 2018. In 2022, the number of ADUs reached 30,000 submitted permits. In that same year, detached single-family home permits were just over 66,000. ADUs can represent a meaningful part of the new housing supply and often provide accessibility to neighborhoods that may have been out of reach.

More than 90 cities and towns have adopted local zoning laws that allow for some form of ADU construction, but strict dimensional and owner-occupancy requirements have led to few being built. In Greater Boston, only 2.5 ADU permits were granted annually per municipality where they were allowed. This mirrors the early experience in Seattle, which initially had restrictive ADU policies but ultimately loosened its zoning laws, including dropping the requirement that property owners live on site. Following the update of its zoning code, Seattle saw ADU permit applications rise dramatically, so much so that ADU construction outpaced construction of single-family homes in 2022.


In March 2022, a bill was introduced on ADUs in the Massachusetts Legislature, but it hasn’t been pushed. The bill as written may not have the desired effect of encouraging construction of ADU units because of language that limits:

▪ Lot sizes. Limiting lot sizes to more than 5,000 square feet may unnecessarily disqualify lots that could support an ADU. Instead, framing the bill based on the percentage of the backyard within the setbacks or proportionate to the primary home has proven to be effective and impactful.

▪ Use case. Limiting use will limit the opportunity to contribute to solving the state’s housing crisis, as ADUs have demonstrated, nationwide, a strong appeal and demand across broad demographic sectors beyond the currently contemplated use cases, which include someone with disabilities or the elderly.

▪ ADU size. The minimum build size, 450 square feet, is an arbitrary number, and high-quality, well-designed ADUs have been built at under 400 square feet.

▪ Owner occupancy. Requiring owner occupancy significantly limits the number of potential properties an ADU could be built on. From 2017 to 2021, 62 percent of single-family homes in Massachusetts were owner-occupied, which means that almost 40 percent of properties wouldn’t be eligible for adding an ADU.


▪ Parking requirement. Not requiring additional parking if located within a half mile of transit would be an excellent way to build moderate density housing and reduce car use.

Statewide ADU policy can more effectively and efficiently enable ADUs and provide a framework for municipalities to successfully address their housing needs without disrupting the character of their communities. It can also provide a framework for municipalities to seek support from the state to achieve their housing goals.

Our team at Bequall anticipates that the national ADU industry will grow into a more than $30 billion sector by 2030. As an entrepreneur and former executive at MassChallenge, the startup accelerator in Boston, I envision that the ADU industry could become a large new industry in Massachusetts that would attract innovators and provide an important growth opportunity for the construction and building industries. A comprehensive statewide policy could effectively dovetail with initiatives already spearheaded by Governor Maura Healey, including the Community Climate Bank, a pioneering effort to finance environmentally sustainable and affordable housing upgrades.

In the face of needing to build nearly 200,000 housing units in Massachusetts over the next eight years, the need for statewide policy change is not just urgent; it’s critical. Cities like Cambridge, Boston, and Newton need 77,000 units combined. Massachusetts ranks 14th nationally for housing undersupply, which puts sustainable economic growth at risk; this key metric must be reduced.


Bequall has analyzed every single-family home parcel in Massachusetts and identified more than a million that could accommodate a detached 400-square-foot accessory dwelling unit, with 375,000 of those properties within a half mile of mass transit. The opportunities are immense if the state allows attached ADUs or garage conversions. One of the keys to ending the housing crisis is hiding in plain sight in our backyards.

Scott Bailey is cofounder of Bequall, which specializes in building single-family studio housing.