Homeowners in Santa Clara County are living in their own houses at the lowest rate seen in more than two decades, new data shows, even as Silicon Valley faces a complex housing crisis that has pushed many out of the region or onto the streets due to a shortage of homes and spikes in housing costs.

The data comes from the Santa Clara County Assessor, which compiled the information based on the number of people claiming a homeowners exemption that typically offers a $70 reduction in property taxes for owner-occupied homes. But over the past nine years alone, the number of people claiming the exemption has dropped by 26,690 homes, according to Santa Clara County Assessor data.

And while the raw numbers of owner-occupied homes are fairly easy to track through the homeowner exemption claims, the reasons behind the trend are harder to nail down, said Larry Stone, Santa Clara County assessor. Even so, Stone, along with market experts and housing advocates, say they have ideas about many of the reasons behind the shift.

Among those factors: A growing number of vacant homes in the county, investors opting to turn their properties to Airbnbs and high living costs pushing people to live elsewhere, even if they own a home.

Proposition 13, the 42-year-old state law that caps property tax increases for homeowners until they sell and has had ripple effects on every part of the real estate market, is also a factor, Stone said.

“People aren’t selling their homes, they will rent them,” he said, noting a single family home in the county can fetch thousands a month. “Less sales means people are still moving, but they’re not selling their homes because there’s tremendous appreciation that they’ve got.”

The assessor’s report also notes that the shift may be partially driven by “a trend by Millennials to rent rather than buy.”

And while across the country, Millennials have eschewed home ownership at higher rates than past generations, homes that do go up for sale in the Bay Area tend to sell quickly, said Sandy Jamison, owner of real estate firm Tuscana Properties and board president of the Santa Clara County Association of Realtors.

“If the housing is priced right, at market, and it is clean and showable, there is no reason it won’t sell within a month,” she said.

But Jamison said many young people still feel priced out of the market, believing they can’t afford to buy a home in the county while also struggling to save for a down payment due to high rent costs.

However, for those who have their homes, she’s seeing trends like people moving out of state and holding onto their Bay Area property — sometimes for years — due to family or business ties in the region.

“They kind of take their time on that transition now,” she said.

Some longtime homeowners who don’t sell their homes after a move may also be putting it off because of large capital gains taxes, which are the taxes on the increased value of the home.

Another trend Jamison is increasingly seeing are investors who opt to turn their properties into an Airbnb or what is known as a “Sober Living Environment” instead of offering a lease.

“Under Airbnb and SLE model, they don’t have to worry about rent control laws because those houses are under a license, and not under a lease,” she said. “It’s more work to run an Airbnb model .. but there’s a high demand for them and they can be quite lucrative if you know how to work that space.”

Meanwhile, tens of thousands of homes are simply sitting vacant in Santa Clara County, according to a recent analysis from Lending Tree, an online lending marketplace that used the U.S. Census Bureau’s 2017 American Community Survey to pull data.

Lending Tree reported last year that more than 28,800 homes were vacant in the Sunnyvale, Santa Clara and San Jose metro area. Some of those vacant homes were accounted for in the tally as currently up for sale or recently sold, seeking a renter or those rented and not yet occupied. Some of the vacant homes are recreational homes or homes for migrant workers. But more than 8,800 vacant South Bay homes came without an explanation in the study.

Those numbers are hard for housing advocate Alex Shoor to swallow.

“In this housing market the amount of value from your home just being there has been extraordinary the last eight years,” Shoor said. “But that emptiness is what is creating this emptiness in our community, including people not having homes, or having to live on the streets, or not coming here in the first place, or being pushed out because they can’t find a place.”

Shoor sits on the San Jose Housing and Community Development Commission and last year, along with Commissioner Huy Tran, proposed the city implement an empty home tax to push property owners to lease or sell the homes not in use, though no initiative has been pushed forward to date.

Still, he hopes to see movement on such a tax that would go to fund affordable homes in the future.

“How many millions of dollars does it take to build more housing?” he said. “You could take money and create a revenue stream from people who are leaving their homes empty and use it to create affordable housing.”

Contact Janice Bitters at [email protected] or follow @JaniceBitters on Twitter.