Progressives are excited about an initiative to change Proposition 13 that could generate billions of dollars every year for schools and local government — and it’s already qualified for the November 2020 ballot.

The California Democratic Party backs the idea, and so do 54% of likely voters, according to an April survey by the nonpartisan Public Policy Institute of California. All it would take to unleash that new waterfall of tax revenue would be to reassess commercial and industrial properties in California every three years instead of whenever they are sold.

But that may not be as easy as it sounds.

Assessing the estimated 642,502 commercial properties that would be eligible for an adjustment in California would be a logistical and legal nightmare, and the state might not see much more than a trickle of new revenue for years, say the people who would have to do the grunt work: county assessors.

Assessors say it is already hard enough to hire and train people to determine the value of commercial property for tax purposes. An independent assessment conducted for the California Assessors Association estimated it would cost counties $470 million to staff up for the increased workload should voters pass the Prop. 13 reform.

Plus, assessors expect deep-pocketed corporations to bulk up their legal teams so they can endlessly appeal the new assessments, jam the new law in the courts, and even hire away assessors’ staffers at higher salaries to help them fight counties.

“It would be impossible to implement,” said Larry Stone, the Santa Clara County assessor.

Stone is no fan of Prop. 13. He said he voted against the 1978 ballot measure because he didn’t think it taxed commercial property fairly, and he still thinks it’s a bad idea: “I don’t think you could devise a more unfair way to calculate property taxes.”

But Stone says the possible 2020 ballot measure is a poor fix.

Under Prop. 13, both residential and commercial property is reassessed only when it is sold. Otherwise, assessment increases are capped at 2% a year.

Many homes change hands every few years, so they’ve been repeatedly reassessed since 1978. Large businesses, however, often remain under the same ownership for a long time. Some California businesses are paying property taxes based on assessments that haven’t changed in 40 years.

Stone cited a 25.9-acre parcel of land in Santa Clara that is owned by Intel. The assessed value of the then-empty property before Prop. 13 passed was $2.9 million, he said. Now the land has buildings on it and is assessed at $23.9 million.

If it were to be reassessed today, Stone estimates, it would be valued at $169 million.

The measure that has qualified for next year’s ballot would not touch the residential half of Prop. 13, something that has been politically sacrosanct in California since the day the initiative passed. But proponents say it would repair a feature of the initiative that has increasingly put the property tax burden on residential owners and would help fund public education.

“With California schools ranked 41st in per-pupil spending and almost dead last in student-to-teacher ratios, we know that the status quo is not good enough,” said Tyler Law, spokesman for Schools and Communities First, the coalition of labor and civil rights groups backing the measure. “Given that nearly every other state successfully taxes commercial property at fair market value, it’s clear that we can do it here in California.”

The measure could raise up to $11 billion annually for schools and local government, including $2.4 billion for Alameda, Contra Costa, Marin, San Francisco and San Mateo counties, according to a 2018 study by the USC Program for Environmental and Regional Equity.

Roughly half the revenue raised by the initiative would go to California’s public schools and community colleges, and the rest would be allotted to cities and counties. Agricultural land and businesses with 50 or fewer employees would be exempt from the reassessments.

The League of Women Voters and a coalition of more than 300 community groups and labor organizers united last year to back the ballot measure, colloquially known as the “split roll” for how it treats commercial and residential properties differently.

“Californians now have the opportunity to reform a 40-year injustice,” Helen Hutchison, president of the League of Women Voters of California, said when the measure qualified for the ballot.

But the organization representing the state’s assessors — while not taking an official position on the measure — said it is worried about how its members would fulfill the initiative’s demands.

A California Assessors Association report conducted by Capitol Matrix Consulting on the split roll’s effect predicts that 75% of all new assessments could be appealed.

Fighting those appeals takes time, money and personnel. Assessor Jeffrey Prang of Los Angeles County, which has $436 billion worth of commercial property — the most in the state — says he has a lot of “logistical” concerns should the measure pass.

Prang said that even though Los Angeles County’s assessor’s department is four times as big as the state’s next largest, with 1,400 employees, it would be overwhelmed for years. It already has a backlog of roughly 25,000 appeals.

“We don’t have the internal capacity to hire 500 appraisers in one year or even five years,” Prang said. “Even if they threw money at me, I couldn’t do it.”

San Francisco Assessor Carmen Chu said her office typically does 1,500 commercial appraisals a year. “If split roll passes, it could be anywhere from 7,000 to 9,000 a year,” she said. It would be a challenge hiring enough staffers to keep pace — especially for a job in high-priced San Francisco during a time of low unemployment.

“When we have a bad economy, government has great time recruiting,” Chu said. “When the opposite is going on, not so much.”

Typically, it is difficult to fill the 130 assessor office jobs that are available annually in California. If the split-roll measure passes, 900 new positions would be required statewide, according to the report done for California Assessors Association by Capitol Matrix Consulting. Even after assessors are hired, it typically takes three to five years to train them before they can appraise major commercial properties.

The challenge of hiring more assessors is even tougher in smaller, more rural places like Calaveras County in the Sierra foothills, said Leslie Davis, the assessor there. She recently hired an entry-level assessor after having spent a year trying to recruit one. She also has openings for more senior assessors — and no applicants.

“We don’t know how we can gear up fast enough for this,” said Davis, who is also president of the California Assessors Association. “It would be chaotic. But if the taxpayers approved this (ballot measure), we would find a way to get it done.”

Proponents said they worked with assessors to write the measure. It provides that counties will determine how to pick up the administrative costs associated with the change.

“The reality is that all meaningful reform requires a healthy transition period, and we remain committed to working with all stakeholders to protect California homeowners and provide funding for our schools and vital services,” said Law, the spokesman for Schools and Communities First.

Stone, the Santa Clara County assessor, has other worries. He fears that when some Silicon Valley corporations get their new reassessments, they will hire his top staffers and pay them much more than they are making in local government to work for them — and fight their new tax bills.

Typically, Santa Clara County fields about 3,000 appeals a year. Stone estimates that could zoom up to 25,000 if the measure passes.

“If you were a company and looking at having to pay $23 million more in taxes, how much would you spend on $2,000-an-hour lawyers and expert witnesses and appraisers?” Stone said. “Where would you get some of those people? For some of them, you’d come into my office and say, ‘How about we quadruple your salary?’”

Alameda County Assessor Phong La isn’t waiting to see whether the measure passes. He is creating an appraisers academy where he will begin training potential staffers. He estimates that hiring people to implement the split-roll initiative would require adding $5 million a year to his $27 million budget.

“If split roll comes down,” he said, “I’m going to need a lot more help.”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli