The assessed value of all taxable property in the Bay Area rose to $1.8 trillion for the 2019-20 fiscal year, up 6.6% from last year, according to county assessors who released their annual rolls over the past week.

Growth rates ranged from 8.1% in San Francisco, thanks to a continuing surge in new construction, to 4% in Sonoma, which is recovering slowly from the 2017 fires.

For the Bay Area as a whole, this year’s growth rate was below last year’s 7.8% increase. Some assessors said a slowdown in home sales led to a slight slowdown in their growth rates. Although the median price of all Bay Area homes and condos sold last year rose about 11% from 2017, the number sold dropped to 81,131 from 86,738, according to CoreLogic.

San Mateo, Santa Clara and San Francisco got big boosts from new construction and land purchases by companies such as Facebook, Google, Genentech, Gilead Sciences and Salesforce.

Counties count on new construction and property sales to keep their property taxes growing. Under Proposition 13, real property in California is assessed, generally at market value, when it is constructed or (with some exceptions) when it changes hands. In between transfers, it can go up only by an inflation rate capped at 2% a year, plus the value of major improvements or additions. That inflation bump can be a significant contributor to roll growth for some counties.

The roll is the assessed value of all taxable real and personal property in a county as of Jan. 1 each year. It’s the amount subject to property tax, which averages 1.2% in California, including voter-approved local taxes. Assessed value is usually less than market value, often far less. Assessors have until July 1 to release their rolls for the fiscal year beginning that date.

The roll includes real property — land, homes and buildings — along with boats, aircraft and business property such as office and manufacturing equipment. It excludes tax-exempt property owned by hospitals, colleges and other nonprofits; and property that is not taxable, including most government property.

Property taxes go to schools, county programs such as sheriffs, jails, courts and social services; and to cities within each county.

San Francisco’s roll grew to $276.9 billion, up 8.1% from last year. “Active construction activities contributed most to the growth. Among the top ones are Salesforce Tower with $355 million added in value and the Warriors’ Chase Center arena with $334 million added in value,” a spokeswoman for the assessor said in an email.

San Mateo County’s roll grew to $238.4 billion, up 7.1%. That was a little below last year’s 8% growth rate, mainly because of a slowdown in residential sales, said Terry Flinn, San Mateo County special assistant to the assessor.

Rolling along Over the past week most Bay Area county assessors have reported their roll for fiscal 2019-20, which started July 1. This is the assessed value, as of Jan. 1, of all homes, land, buildings and other property subject to property tax. It’s not the same as market value, which is usually much more. County 2019-20 roll (in billions) Change from last year Alameda $308.8 7.1% Contra Costa 215.2 5.3 Marin 82.2 5.1 Napa 41.9 6.6 San Francisco 276.9 8.1 San Mateo 238.4 7.1 Santa Clara 516.0 6.8 Solano 58.0 5.5 Sonoma 92.6* 4.0* Bay Area total 1,830.0 6.6 *Estimate Source: Bay Area county auditors, Chronicle research

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Gilead Sciences added $449 million to the roll, most of that from the construction of two buildings at its Foster City headquarters. The next biggest contributors were Genentech ($446 million), Facebook ($271 million), Facebook subsidiary Hibiscus Properties ($214 million) and Google, which added $165 million in Redwood City and San Bruno. Together, Facebook and Hibiscus were the biggest contributor, thanks largely to the completion of MPK 21, its new Frank Gehry-designed headquarters in Menlo Park.

Genentech, despite its unrelenting property-tax appeals, overtook United Airlines as San Mateo County’s highest-assessed taxpayer. Genentech’s properties are now assessed at $2.7 billion, followed by United at $2.4 billion and Gilead at $2.2 billion. United’s most valuable property is its aircraft, but it also owns a giant maintenance facility.

Santa Clara County’s roll rose 6.8% to $516 billion, biggest in the Bay Area by far. It can thank Google for its $759 million addition to the roll, and Apple for adding $271 million; together they accounted for 3% of the county’s increase. Apple’s increase was smaller than last year’s, when it added $1.5 billion from construction at its “spaceship” headquarters and the purchase of office buildings and business property.

Assessor Larry Stone said he’s often asked to compare the county’s building boom to the one in the 1990s dot-com era. He said the previous one “was largely speculative, builders were building spec buildings to lease primarily to startups and young companies with no earnings. When we went from the dot-com boom to the dot-com bust, some tenants went belly up or walked away from their lease obligations.”

Today, he said, “Apple, Google, LinkedIn, Nvidia are buying land and building buildings that they occupy. That is a different type of stability going forward that we never had before.”

He added that San Jose is finally getting the kind of high-value commercial and industrial development that in the past went to places like Santa Clara, Sunnyvale and Mountain View. After languishing in the bottom third of the county’s cities in terms of roll growth, now it’s in the top third, he said.

Google has been gobbling up property in downtown San Jose, some from the city at prices far above their previous assessments. Although its purchases have sparked protests, “the city made a ton of money” without giving Google any concessions, Stone said.

Marin County’s roll grew 5.1% to $82.2 billion. Some of the county’s largest property sales didn’t contribute much to its roll growth and one even detracted, Assessor Shelly Scott said. The Embassy Suites hotel in San Rafael sold for $37.9 million, not much ahead of its prior assessment of $34.3 million.

The biggest residential sale was an estate at 800 Corte Madera Ave., once owned by the late concert promoter Bill Graham. It sold in July for $21.4 million, below its previous assessment of $22.5 million, Scott said.

The sale of two office buildings in Novato did bolster the roll, by $18.6 million.

Alameda County’s roll grew 7.1% to $309 billion, “mostly from general growth in real estate,” said Assessor Phong La.

Contra Costa County’s roll grew by 5.3% to $215.2 billion. “We’re not like Santa Clara County, where they have Apple and Google (building like crazy). That’s Larry Stone’s gift from God,” said Contra Costa Assessor Gus Kramer. “We’d like to be the industrial giant we were in 1968, but we’re not,” he added. In that year, 39% of the county’s tax base was commercial/industrial. Today it’s 6% to 8%, he said.

Rising home prices in towns such as Brentwood and Oakley, which were hit hard during the recession, helped Contra Costa’s roll grow. When the market value of a home falls below its assessed value, homeowners can request a temporary reduction in assessed value. This is known as a Proposition 8 reduction, and during the recession, hundreds of thousands of Bay Area homes got them, some automatically.

As prices recover, the assessor can raise the assessment of a home in Prop. 8 status to its market value, even if it exceeds 2% a year, until the assessed value reaches the point it would have been had it never gotten the Prop. 8 reduction. This restoration in value can add significantly to the roll in some counties.

Contra Costa County still has about 20,000 homes with a Prop. 8 reduction, down from a peak of 190,000. By comparison, San Mateo County has only 245 homes in Prop. 8 status, down from 34,700 in 2011-12.

Solano County still has about 8,900 homes in Prop. 8 status, down slightly from 11,000 last year. The Solano real estate market “is not progressing greatly. There are a lot of homes for sale with high listing prices, but we are not generating a lot of sales,” said Lance Houser, Solano’s assistant assessor. Solano’s roll grew 5.5% to $58 billion.

Sonoma County has requested an extension for its roll but expects it to rise 4% to $92.6 billion. The October 2017 wildfires destroyed about 5,300 properties and wiped $1.8 billion off the county’s roll last year. As of Jan. 1, only about 130 properties had been replaced in Santa Rosa, although the city expects another 2,000 completions by this time next year, said Greg Walsh, the county’s chief deputy assessor. When those properties are replaced, the assessor restores them to their pre-fire value, and does not reassess them unless they have a new owner.

Napa County, which lost about $500 million in assessed value because of the fires, fared a little better thanks to some winery and vineyard sales. Its roll rose 6.6% to $41.9 billion. Napa County Assessor John Tuteur said the real estate market there “is cooling off a bit. I think we are going to plateau for a while.”

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