IF THE SENATE has its way, single-family zoning in Massachusetts may soon find its days numbered.

Two years after a sweeping housing bond bill declared small accessory dwelling units could be built in any single-family district, the state Senate wants to double down on zoning up.

The chamber’s economic development bill, which lawmakers have teed up for a vote next week, includes a provision that would require municipalities to allow duplexes as-of-right on all residentially zoned lots, meaning the projects would not require special variances. These projects would still be “subject to reasonable regulations” on septic system access, site plan review, and design factors.

State Sen. Julian Cyr, whose hometown of Truro just advanced a similar policy through its zoning task force, said the duplex zoning proposal is “a simple but incredibly meaningful reform.”

“It’s making the housing that we’re providing more affordable,” Cyr said. “It’s making new construction of smaller-scale housing more within reach for residents in Massachusetts, especially this next generation, where we really can’t afford to live in the state because of decades of lackluster housing production,” said the 40-year-old lawmaker.

Undoing the right to have an area zoned strictly for single-family homes would also represent a seismic shift in state law, taking aim at a sacrosanct pillar of suburban zoning codes.

Almost 60 percent of Massachusetts’s roughly 3 million homes are single-family houses, according to the state. Beacon Hill has been working to squeeze into almost every legislative vehicle policies that can help meet the goal of building 222,000 new housing units by 2035 to meet pent-up demand.

Housing advocates have been calling for greater “infill” density in areas that can support it but have been held back because of strict zoning requirements. The duplex move, they say, is promising but still shy of their dream scenario.

The Senate proposal is an “important provision,” said Jesse Kanson-Benanav, executive director of Abundant Housing Massachusetts. “We have an extreme shortage of homes, throughout the Commonwealth, that’s driving this cutthroat competition for housing. We really need every strategy to get the various homes at various price points that we need.”

But groups like Abundant Housing are looking for more. Kanson-Benanav said his organization has been pushing new statewide zoning allowing up to five units on a lot as-of-right, where there is adequate utilities access. He pointed out that states like Maine and Vermont allow up to four units as-of-right on lots with access to water and sewer.

A House version of the economic development bill, passed last week, included a handful of housing-oriented initiatives like supporting the conversion of commercial properties into new multifamily and mixed-use housing, and clear definitions and timelines around site plan review, which is the process for determining whether a proposed development conforms to local regulations.

It also folded in a so-called “Yes in God’s Back Yard” policy, which backers say could potentially lead to hundreds of thousands of new housing units by reducing local zoning barriers for parcels owned by religious communities.

The duplex provision introduced in the Senate’s version is something of a spiritual heir to the 2024 housing bond bill that legalized accessory dwelling units in all single-family districts.

Lawmakers have been reluctant to make sweeping zoning changes in the years since the MBTA Communities law ushered in a requirement to rezone for multifamily housing in cities and towns served by the MBTA system. While most municipalities have complied, the law generated fierce blowback from dozens of communities, and several lawsuits, as local officials decried the 2021 law as an ungainly assault on local control.

Municipal pushback also dogged the ADU rezoning.

It’s not clear whether the House will have any appetite for the sweeping duplex change, and it’s already drawing some familiar opposition from municipal leaders.

“While well-intentioned, a statewide by-right duplex mandate would represent a significant erosion of municipal authority over local planning and zoning,” said Adam Chapdelaine, executive director of the Massachusetts Municipal Association, which advocates for cities and towns. “The MMA and municipal officials share the goal of creating more housing, but durable solutions are only achieved through partnership with communities, not by overriding local decision-making.”

Business leaders, on the other hand, have been pushing for more housing to make the state more affordable to the young workers fleeing to cheaper New England neighbors.

The Greater Boston Chamber of Commerce released a report this week recommending, among other things, allowing multi-family housing by right.

Without having seen the final language, Chris Eicher, senior vice president of public policy and government relations at the chamber, said duplexes by right would be loosely consistent with that recommendation, and “I do think statewide solutions, and particularly in the realm of zoning, are necessary.”

While progress on ADU permitting and construction is slower than the Healey administration had initially hoped for, researchers and the administration say there is clear demand for the small accessory units.

This duplex provision, Cyr noted, is bridging the gap between the ADU goal of making more efficient use of existing lots and looking toward future development.

It would apply to a person who wants to divide an existing lot, or build a second floor on a single-family house — as long as the final building was still consistent with the reasonable design rules and character of the municipality. But it is more oriented toward new construction.

Anticipating the pushback that would come from the loss of single-family zoning in Massachusetts, advocates insist it’s not all that dramatic a difference from the status quo and it won’t spell the end of stand-alone one-family homes.

“Really the only aesthetic difference between single-family and a duplex is there’s two doors, not one,” Cyr said.

And in places where single-family housing makes the most sense for the market — like some suburbs or more rural areas — Kanson-Benanav says “it is not going away. I do not expect to see our lowest density communities, our most rural areas becoming full of duplexes.”

But it might unclog a pipeline of housing for people who would happily rent or buy a shared building if the option were available.

“This is about choice,” Kanson-Benanav said.