For anyone who has contemplated California’s devastating housing shortage and wondered how the state got in this mess, here’s one short answer: Senate Bill 106.

Of course, this single unfortunate piece of legislation is not responsible for a crisis rooted in decades of bad state and local government decisions. But the bill, which would exempt Marin County from housing density standards that apply to the rest of the Bay Area for more than 10 years, does embody the sort of parochial policymaking that has been ruinous for the state’s greater good.

Pushed by Assemblyman Marc Levine, a Marin County Democrat who secured a similar development dispensation for the state’s wealthiest county in 2014, the bill would extend the exemption, currently scheduled to expire in 2023, to 2028. It would continue to classify the county, including its largest cities, San Rafael and Novato, as suburban, excusing it from the high-density housing developments prescribed for metropolitan areas.

The Legislature’s ruling Democrats introduced the exemption, as the Los Angeles Times reported, as part of a budget trailer bill addressing a wide variety of unrelated issues and escaping the level of public scrutiny applied to most legislation. Now before the state Senate, it was passed by the Assembly this week without a single Democratic or Bay Area lawmaker voting against it. David Chiu, D-San Francisco, who abstained, was the lone member from the housing-starved region who did not vote for the bill.

“Every part of our state must create housing to help solve the affordability crisis,” Chiu said in a statement, “but this policy makes that harder.”

While it neighbors San Francisco and is considered part of the same metropolitan region for census purposes, Levine said that Marin County is more like its northern neighbor, Sonoma County, and that it was therefore a mistake to subject it to the density requirements that apply to the rest of the Bay Area. Because suburban-style housing is more likely to be built in Marin County than “San Francisco- or Oakland-style developments,” the legislation is pragmatic, Levine said. “Less density doesn’t mean less housing.”

But it’s a result of policy, not an accident of geography, that the median price of a single-family home in Marin County is $1.25 million, the highest in the region. The county’s population density of about 500 residents per square mile is a third of San Mateo County’s and a fourth of Alameda County’s.

Marin is an extreme but by no means unique example of the forces that created the housing crisis. As long as local objections to development carry the day, and Sacramento lacks the courage to override them, homelessness and other repercussions will persist.

Democratic legislators have often noted that they are considering more than 100 bills addressing the housing crisis. Sen. Scott Wiener, D-San Francisco, and Gov. Jerry Brown have made particularly promising proposals to reduce barriers to development. But the only one being rushed through the Legislature at the moment would have the opposite effect.