This may be hard to believe considering how far home prices have risen, but there are properties in every California county — including almost 50,000 in the Bay Area — that are still benefiting from temporary property tax reductions they got during the housing bust.

That’s because the market value of these properties, which are mostly residential, is still below their “Proposition 13 value.” This is where they would be assessed today under Prop. 13, had they not gotten the temporary reduction.

These properties make up a small percentage of the tax base in most Bay Area counties except Contra Costa, where they number 20,920, or 18% of all parcels. San Francisco has the smallest number — 617, or 0.3% of all parcels. Of those, 369 are condo units in the Millennium Tower, where values have sunk along with the building.

Statewide, they make up about 8% of all secured parcels, according to a survey by the California Assessors’ Association. In the Bay Area, they account for just under 2%.

In most counties, they tend to be homes purchased around 2005 to 2007 in areas that were undergoing rapid development and were slower to recover from the housing bust. Think of places like Brentwood and Antioch in Contra Costa and American Canyon in Napa County.

Under Prop. 13, properties in California are generally reassessed for property tax purposes only when they are sold. Assessed value is the amount subject to property tax.

In between sales, the assessed value can’t go up by more than an inflation factor capped at 2% a year, plus the value of additions and major improvements. This adjusted value is called the “factored base year value,” or Prop. 13 value for short.

Although Prop. 13 limited taxes on the upside, Proposition 8 allows for a temporary reduction in a property’s tax assessment when its market value drops below its Prop. 13 value. Properties with these reductions are said to be in “Prop. 8” or “decline in value” status. Reasons for a reduction could include damage by fire or flood, or a downturn in the overall market.

Once a home has gotten this reduction, the county assessor must reassess it at market value as of Jan. 1 each year — which can result in a further decrease or an increase greater than 2% a year — until it reaches its Prop. 13 value, or where it would have been without the temporary reduction. At that point, “all the protections of Proposition 13 kick in,” said Santa Clara County Assessor Larry Stone. It becomes subject to the 2% annual limit and reassessed only when sold.

Normally, owners must apply for a reduction, but after home prices started falling after 2007, many county assessors applied them automatically to wide swaths of homes purchased near the peak. Most homes didn’t get them because their market values remained above their Prop. 13 values.

Statewide, the number of properties with these reductions soared from about 335,000 in fiscal year 2007 to a peak of nearly 3.6 million in 2012, then dropped to about 1 million in 2018, according to the California Board of Equalization data. It doesn’t have data for this year, but, according to the association’s survey, it’s about 975,000.

In the Bay Area, the median price for homes and condos surpassed its mid-2007 peak of $665,000 in April 2016, then went on to post many new highs, topping out at $875,000 in June 2018, according to CoreLogic. In June, it was $856,000.

Despite this strong rebound, some homes are still in Prop. 8 status because of the inflation factor.

Suppose someone bought a home for $1 million near the peak in 2007 and later got its assessment reduced. Today, its Prop. 13 value would be about 20% higher, or $1.2 million, because of the inflation factor, said Brian Hitomi, Alameda County’s chief deputy assessor.

If the home was worth $1.1 million on Jan. 1 of this year, it would be assessed for $1.1 million, but it would still be in Prop. 8 status — and subject to annual reassessment — because it hasn’t reached its Prop. 13 value.

Alameda County still has about 5,000 properties with a temporary reduction, down from about 108,000 in 2011-12. Some of the 5,000 “have condition problems, but the vast majority were purchased at the peak,” Hitomi said. During the downturn, these reductions took a bite out of property taxes, but today the impact is negligible because they are close to being fully restored.

Solano County still has about 8,900 properties with a temporary tax reduction, or 6% of its total. Some homes that sold for more than $1 million at the peak “now are probably in the $700s. They have quite a ways to catch up,” Assessor Marc Tonnesen said.

Those properties are costing the county money, but at least it has the opportunity to recapture revenue as their market values go up. During the downturn, a lot of people walked away from their homes. When those buildings changed hands, they were reassessed at depressed levels. The county had no opportunity to recapture that value until they were resold at higher prices.

San Mateo County has 1,017 properties with temporary reductions, or 0.5% of all properties. Of those, 245 are residential, 37 are commercial and the rest are mainly mobile homes, apartment buildings and vacant or agricultural land.

In Santa Clara County, 1,843 properties, 0.4% of the total, still have reductions. That’s down from 136,559 properties, or nearly 25%, in 2012-13, Stone said. About 1,400 of the remaining properties are residential, and of those, 43% are in Gilroy and Morgan Hill, which combined have less than 4% of all parcels in the county.

In Sonoma County, 6,641 properties, or 3.6% of the total, still have temporary reductions. Only 982 of those were damaged or destroyed by the 2017 wildfires that consumed more than 5,000 homes in the county. That number is not higher because many fire properties were sold and reassessed, or they were purchased so long ago that their Prop. 13 value is still below market value, said Chief Deputy Assessor Greg Walsh.

In Napa County, 2,242, or 4.3%, of all properties have a reduction. “We aren’t putting many if any (new ones) on Prop. 8, but they are slowly dropping away. Now that the market may be stagnating, they may stay around for a few years,” Assessor John Tuteur said. “In the 1980s, we put a bunch of condominiums on Prop. 8. They stayed on Prop. 8 for 15 years until the market finally caught up.”

In Contra Costa, most of the properties with reductions “are in a select part of Richmond, Oakley and Brentwood, a little in Antioch and Pittsburg,” Assessor Gus Kramer said.

Modoc County, in the northeastern corner of the state, has 54% of its properties with a decline-in-value status, the highest percentage in the state.

About 95% of those properties, however, are in one subdivision called California Pines. “It was built in the 1960s or 70s, but the developer died in a tragic ski accident and all development stopped,” said Modoc County Assessor Kristen DePaul. Over the next several decades, various companies began aggressively marketing the lots, mostly to out-of-town buyers for retirement or vacation homes. The last to do so was National Recreation Properties Inc. of Irvine, which used former “CHiPs” star Erik Estrada as its spokesman.

“They were a hot commodity for years and really inflated the market for this piece of land out in the middle of absolute nowhere,” DePaul said. After NRPI went bankrupt in 2012, “there was just no market” and prices “dropped drastically,” she added. “I really don’t know if the subdivision will ever rebound without another marketing agency behind it.”

Other counties with a significant percentage of properties with temporary reductions include Alpine (30.3%), Butte (18.4%), Calaveras (19.7%), Kern (17.2%), Mono (21.8%), Plumas (24.2%) and Riverside (18.8%).

Kathleen Pender is a San Francisco Chronicle columnist. Email: kpender@sfchronicle.com Twitter: @kathpender