Washington is joining a national trend in overhauling single-family zoning to increase apartment density and address a worsening housing shortage that proponents say has contributed to rising affordability concerns and homelessness.

Gov. Jay Inslee signed House Bill 1110, which overrides local zoning laws and allows duplexes, triplexes and fourplexes in most neighborhoods as part of a package of laws passed by the Washington State Legislature in Olympia aimed at clearing barriers to residential construction. Seattle and other cities will need to add 1 million houses over the next 20 years to keep up with population growth amid a worsening homelessness problem, the Washington Commerce Department estimates.

States, cities and counties around the country have passed laws to shake off zoning that for decades has reserved large sections of municipalities for single-family homes by preventing housing that provides two to four units. These efforts gained momentum during the pandemic when home prices and rents soared to records across many cities around the country and a housing shortage ensued.

Oregon became the first state to ban single-family zoning in 2019. California passed a similar law in 2021 and a law passed by Maine last year is set to go into effect July 1. Montana legislators passed several bills last month to increase multifamily density and create more affordable urban housing. Colorado's legislature is considering a similar overhaul to zoning regulations.

Municipalities across the country making similar zoning changes in recent years include St. Petersburg, Florida, and the Washington, D.C., suburb of Arlington County in Virginia, Minneapolis, Cambridge outside Boston and California's Berkeley, Oakland and Sacramento.

About 75% of the residential zoning across the country is for single-family homes, according to nonprofit Smart Growth America and other advocates for rewriting zoning laws. Single-family zoning dates to the early part of the 20th century as cities sought to exclude multifamily housing generally under the rubric of protecting the public welfare.

"Restrictive zoning that permits only single-family homes is the clear culprit of the housing shortage crisis and the record-high housing prices that we’re seeing across the country," Michael Rodriguez, Smart Growth America director of research, said in an email. "Increasingly, housing costs are out of reach for middle- and low-income people and families, especially in regions and neighborhoods that afford the most opportunity in terms of access to good jobs and quality schools."

Washington's HB 1110 was among 10 bills signed this month by Inslee aimed at boosting housing supply in a state where it’s expensive and sorely needed.

Other legislation signed into law includes House Bill 1337, which makes it possible to permit and build more accessory dwelling units, and Senate Bill 5290, aimed at streamlining local permit and development regulations.

"This was the most significant session for pro-housing legislation that we’ve seen in recent years," said Dylan Sluder, state government affairs manager for the Master Builders Association of King and Snohomish Counties, a construction trade group that supported several of the bills, including the single-family zoning overhaul.

"The crisis demanded action because we're at the point where there's no real entry-level pricing to buy a home," Sluder said in an interview. "It’s been a long time coming, but it goes a great deal toward addressing the housing crisis that our state is facing."

Legislation to roll back single-family zoning in Washington failed in recent years as cities lobbied to keep their local grip on zoning regulations. Rep. Jessica Bateman, a Democrat from Olympia that co-sponsored HB 1110 with Republican Andrew Barkis from Olympia, said both political parties in the House and Senate compromised to craft legislation aimed at adding more housing.

“They said that 2023 was going to be the year of housing, and we did not disappoint,” Bateman said at Inslee's bill signing event. “Over 50 bills were introduced this session related to housing."

Sluder said that while the Washington measures are large steps in the right direction, solving the housing crisis will take continuous work over years.

"There’s multiple slices and no silver bullet for the problem," Sluder said.