JUST AS AN end-of-year lawsuit launched a broadside against a decades-old policy to boost affordable development, state lawmakers moved to make it easier for cities and towns to roll out their version of that same policy.

A Cambridge developer is arguing that the city’s so-called inclusionary zoning program, which requires developers to set aside a number of units for lower- and middle-income people, is unconstitutional at its core. The suit aims to crack the foundations of inclusionary zoning in Massachusetts, adopted by communities like Boston, Cambridge, and Somerville.

Meanwhile, senators struggling to make headway on a housing crunch across affordability levels are looking to pull all the levers they can, including lowering the difficulty of implementing inclusionary zoning locally.

The legal and legislative moves set up a clash over rules governing housing development at a time when there’s broad consensus that the state needs to build more units across communities.

Inclusionary zoning rules condition getting permits to build residential projects of a certain size on making a percentage of the units affordable. Cambridge credits its inclusionary zoning policy with creating over 1,600 units of affordable housing, and Somerville in 2023 described its policy as the “primary pipeline adding to the City’s permanent affordable housing stock.”

But a lawsuit filed last month in Land Court by Cambridge zoning attorney and developer Patrick Barrett and the conservative Pioneer Legal Foundation claims that the Cambridge policy is unconstitutionally forcing developers to “surrender fundamental property rights” by selling their units at lower than market rate.

Cambridge first passed its inclusionary housing policy in 1998. As the Greater Boston housing market surged in the mid 2010s, the Cambridge City Council ushered in one of the highest inclusionary zoning rates in the country — a 20 percent set aside for low- and moderate-income renters and homebuyers for new construction with 10 or more units.

Somerville’s program has been in place since 1990, expanded in recent years to require 20 percent of low- and middle-income units in buildings of four or more units. Boston has managed an inclusionary development program since the Mayor Thomas Menino era in 2000. The city strengthened its requirements in 2024, lowering the number of units needed to trigger the policy and increasing the required number of affordable units.

Housing production has slowed, even in historically strong markets like Greater Boston, which city officials attribute mostly to macroeconomic trends like the COVID-19 pandemic affecting supply chains and labor costs, and spiking inflation.

In a statement when the suit was filed, Cambridge spokesperson Jeremy Warnick said the city’s Inclusionary Housing Zoning Ordinance “has become a model for other communities looking to create affordable housing for low- and moderate-income residents and ensure the socioeconomic diversity of the community is reflected in new housing.”

The lawsuit could take two years or more before a trial even starts. Yet over the course of the challenge, which targets the very nature of inclusionary development, lawmakers may kick open the gates for more communities to consider an array of inclusionary policies of their own.

A showpiece bit of pro-housing legislation, shorthanded as the “YIMBY” bill, got the green light from the Senate’s housing committee in December. The bill would establish a statewide housing production goal, reduce the minimum lot size for new construction, and do away with parking minimums. But the version approved by the committee also includes new language that would allow municipalities to create inclusionary zoning policies by a simple majority vote of their legislative bodies rather than the two-thirds support often required in local codes. An earlier version of the bill, filed by Sen. Brendan Crighton of Lynn, did not include any provisions about inclusionary zoning.

Crighton said inclusionary zoning is an important tool for unlocking affordable housing, and allowing the policy to be adopted by majority vote, not two-thirds support, would bring it in line with recent reforms allowing other zoning decisions to be made by simple majority.

“It’s really difficult to make any of these pro-growth zoning changes with that super majority threshold, which is really not typically how democratic bodies should function,” he said.

Some of the loudest pro-housing voices in progressive Massachusetts cities, however – proponents of “Yes In My Back Yard” building, or YIMBYs – are notably cool to inclusionary zoning policies, which some say can slow growth by driving up developers’ costs.

YIMBYs advocate for robust housing development at all affordability levels, but they embrace the supply-and-demand argument that significantly boosting production is the key to ensuring more lower-cost housing. They tend to oppose or at least voice skepticism of policies like inclusionary zoning that could in any way slow production of new units.

The Somerville YIMBY group, comparing the downside risks of inclusionary development to a game of musical chairs, notes that San Francisco recently rolled back fees and inclusionary zoning rules as its housing market stagnated.

“Inclusionary zoning helps lower-income people get chairs, and that’s great,” the group wrote, “But it also risks stopping the creation of enough chairs for everyone, which would only exacerbate the housing crisis.”

Abundant Housing Massachusetts, the state’s major YIMBY organization, considers the sprawling pro-housing bill a priority because of other provisions. But the new language on inclusionary zoning caught YIMBYs off guard.

“Inclusionary zoning at the right level can be an important tool for creating diverse communities,” said the group’s executive director, Jesse Kanson-Benanav. “But it has to be at the right level, and even in the strongest market we have in Greater Boston – Cambridge – inclusionary zoning without funding or benefits to developers is not working and producing the affordable housing that we would expect.”

A standalone inclusionary development bill more in line with the YIMBY pitch was filed by Sen. Lydia Edwards of East Boston, Senate co-chair of the housing committee.

Under Edwards’s bill, a community could by simple majority vote approve an inclusionary zoning ordinance or by-law that requires that up to 13 percent of units be affordable, so long as it will not “unduly constrain the production of housing in the area impacted.”

Both Crighton’s broader YIMBY bill and the standalone legislation from Edwards were reported out favorably from the Senate housing committee and sent to the Senate Ways and Means Committee.

“I understand there are concerns around unintended consequences from our partners in the YIMBY world,” Crighton. “And I certainly don’t want anything to be signed into law that could actually limit housing or have unintended consequences. So we’re certainly open to different ways that we could prevent this from being misused.”

While the lawsuit plays out that challenges the constitutionality of inclusionary zoning policies, Crighton suggested there is support for expanding inclusionary zoning even if details need fine-tuning.

“I think there’s still a lot of interest there,” he said.