It’s Wednesday, October 26, and San Francisco just took a step to combat the housing and climate crises.

San Francisco’s Board of Supervisors approved a bill last week making it legal to build four units of housing on any residential lot in the city and up to six units on corner lots. This marks a big shift for San Francisco, one of the most unaffordable cities in the country. In spite of its already high population density, much of the 47-square-mile city is zoned for single-family housing.

“The way much of San Francisco is zoned today makes it easier to flip existing housing into monster homes than to build small apartment buildings for regular working people,” said Rafael Mandelman, a city lawmaker, in a February statement introducing the final version of the legislation.

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Housing affordability advocates and environmentalists have long argued that exclusively single-family zoning pushes lower-income families out of the local housing market. This, in turn, exacerbates urban sprawl and makes people more reliant on cars to get around, as low- and middle-income families move further from inner cities to find affordable housing. Across the Bay Area, the region with the nation’s most expensive housing, some 85 percent of residential land is zoned exclusively for single-family homes.

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Making it easier to build multifamily housing by “upzoning” neighborhoods could significantly reduce greenhouse gas emissions and improve public health. Research has shown that doubling urban density, particularly near reliable public transit, could reduce CO2 emissions from household travel by nearly half and residential energy by more than a third. Greenhouse gas emissions from transportation account for 27 percent of U.S. greenhouse gas emissions, more than any other source.

The San Francisco legislation, which Mayor London Breed is expected to sign, builds on a California state law allowing duplexes on single-family-zoned lots that went into effect at the beginning of this year. They’re both part of a nationwide push for more affordable housing and to combat the climate crisis. In 2018, Minneapolis became the country’s first major city to end its decades-old exclusively single-family zoning laws. In 2019, Oregon followed suit, allowing duplexes, triplexes, and fourplexes to be built on residential lots previously restricted to single-family construction.

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