Mass. is getting more granny flats. But it’s still hard to build them. – The Boston Globe

Massachusetts took a big step in 2024 when the Legislature legalized so-called “accessory dwelling units” statewide as part of an effort to rein in the state’s housing crisis. More than a year later, it’s clear that the law is working — but that it also needs tweaks before accessory units can meet their full potential.

These small units, nicknamed “granny flats,” can be constructed in someone’s backyard, or they can simply be renovated third floors, garages, or basements. They’re a popular option for seniors seeking to downsize and families looking for some rental income.

Prior to the state law, some communities allowed accessory units, but many did not. Even among those cities and towns that did tolerate accessory dwelling units, zoning often varied from one municipality to the next, making it difficult for builders who needed to decode each municipality’s rules. Some towns also included unreasonable restrictions, like requiring that only a homeowner’s family members could live in the accessory units.

Housing advocates viewed allowing accessory dwelling units statewide as a “low-hanging fruit” policy — a way to add housing that was relatively cheap and avoided some of the cost and political obstacles that housing measures often encounter. The state legislation also overrode some zoning restrictions, including those that limited accessory units to family, while leaving some other local rules intact.

One year after the law went into effect, this approach has proved fruitful: Towns across the state have approved 1,200 ADU permits and seen even more applications, in some cases up to a threefold increase from previous years.

A study published last week by Boston Indicators (the research branch of the Boston Foundation) and Abundant Housing Massachusetts found that forcing the hand of municipalities on accessory dwelling units accomplished more in one year than 50 years of zoning reform efforts at the local level.

The problem, though, is that municipalities retained too much power. As the study recommends, there should be clear, uniform state regulatory standards for ADUs, with minimal opportunities for municipal-level variation.

“A comprehensive agenda is needed to address regulatory barriers to housing production, spanning building, fire, energy, septic system, wetlands, and stormwater rules,” the study’s authors wrote. “The barriers include the fragmented complexity of the regulatory system itself.”

Making standards more uniform doesn’t have to mean lowering them — it just means moving away from patchwork rules that make it harder for companies to build accessory units at scale.

Chris Lee at Backyard ADUs, a company that designs and builds modular dwelling units in New England, says the report’s findings make sense. The inconsistent interpretations across 350 towns and cities cause builders and engineers to “struggle to design work for the town that will be accepted,” he said. (The state’s 351st municipality, Boston, isn’t covered by the law.)

The potential is significant. The report calculates that if just 2 percent of single-family homes in Massachusetts added an accessory unit, the state would see more than 30,000 new homes that advocates say are generally more affordable. Building an accessory dwelling unit inside a pre-existing house can cost between $75,000 and $100,000; and a detached unit usually runs between $250,000 to $350,000, making them much more affordable than purchasing a single-family home in most regions of the country.

“For developers of missing middle housing to benefit from an economy of scale, they have to undertake many projects, across jurisdictional lines,” according to the study. “The ADU case study has shown just how challenging this is.”

Lee estimated that he could reduce up to $30,000 of preconstruction costs such as surveying and architecture if his company could work with consistent regulations across towns, which he said could enable them to double their production.

Streamlining permitting for accessory dwelling units isn’t a panacea. Landlords still must be willing to actually build them and rent them to long-term residents. Retirees must believe it’s worth downsizing to one. But the fact that so many have been permitted over the last year point to the clear demand and makes the case for policy makers to keep refining the law.

There is precedent. California, for example, had an equally ambitious goal but has blown past it, going from only 1,300 permits approved its first year to more than 30,000 nine years later. “It is important to understand that California did not accomplish its ADU outcomes with one legislative reform,” the study’s authors wrote. “California’s success required sustained legislative attention.”

Massachusetts should be able to realize those kinds of results too. Conversely, if even the “low-hanging fruit” of zoning reform falters in the Commonwealth because of local red tape, then the state has bigger problems ahead to solve its housing crisis.

Editorials represent the views of the Boston Globe Editorial Board. Follow us @GlobeOpinion.