San Francisco Assessor Carmen Chu is suing both the San Francisco Giants and the city’s own Assessment Appeals Board over a multimillion-dollar property tax assessment break granted to the team’s Oracle Park.

At issue is the assessed taxable value of the 42,000-seat waterfront stadium, which sits on land leased from the Port of San Francisco.

Chu set the park’s assessment at $415 million for 2015, $421 million for 2016 and $430 million for 2017.

The Giants countered with an estimate of $309 million for 2015, $306 million for 2016 and $298 million for 2017.

Why the difference? The assessor’s office argues the ballpark is like a house or office building — its value has risen during the real estate boom. But the Giants argue that the ballpark may need to be upgraded in the future — so it was less valuable.

After hearing both sides, the Assessment Appeals Board, which is independent of the Assessor-Recorder’s Office, decided the park’s value had risen, but at a far slower rate than set by the assessor. The board set the park’s value at $385 million for 2015, $405 million for 2016 and $437 million for 2017.

The difference would mean San Francisco losing out on about $543,000 in property taxes over the three years in dispute.

The taxes for 2018 are still under appeal.

“A single legal error in the board’s analysis caused a reduction of approximately $185 million each year,” the assessor states in the lawsuit, filed Friday in San Francisco Superior Court.

“The single error is that they double counted the current depreciation of the ballpark and gave credit for future depreciations that may or may occur in the future,” said Vivian Po, spokeswoman for the assessor. “We don’t want the Giants to get property tax deductions that nobody else can get.”

Assessment Appeals Board Administrator Dawn Duran stands by the decision.

“The board handles all of the large conflicts, including the big hotels and office buildings and the three who heard the (ballpark) case are all seasoned board members,” Duran said. “I really don’t have a comment, as I have yet to see the lawsuit,” she added.

The Giants didn’t respond immediately to a request for comment.

As for the oddity of one city office filing suit against another, Duran said, “It’s rare but it does happen. The assessor has the right to file suit, just as any taxpayer has the right to take (the board) to court.”

It’s not the first tax go-round among the assessor, the Giants and the appeals board. Since Oracle Park opened in 2000 as Pac Bell Park, the Giants have consistently challenged the city’s annual tax assessments.

For example, Chu set the 2014 value of the ballpark at almost $407 million, while the Giants pegged it at more like $158 million and then upped the estimate to $254 million just before the hearing.

The Assessment Appeals Board eventually ruled that the ballpark’s value was about $365 million, way below the assessor’s estimate, earning the Giants at $548,343 refund.

Oracle Park isn’t the only stadium where different parties see the tax valuations differently.

Santa Clara County Assessor Larry Stone sued his county Assessment Appeals Board in late May for its assessment of the 49ers’ Levi’s Stadium in Santa Clara.

That board ruled the 49ers were responsible only for half of the assessed value on the $1.2 billion Levi’s Stadium, because the football season is only half the year long.

Stone argued that the 49ers use the facility year-round for concerts and other sporting events “that have a value that the Assessment Appeals Board is ignoring.”

The board didn’t buy the argument and, as a result, the 49ers got a one-time, $36 million tax refund and a $6 million tax cut in their annual taxes.

Meanwhile, the Santa Clara suit is making its way through Superior Court.

“These sports stadium tax deals are quite complex, and teams all across the nation are paying significant attention to these cases,” Stone said.

Triple play: It was a good night for San Francisco Mayor London Breed and a bad night for the progressives at the Democratic County Central Committee Wednesday evening.

Not only did the committee vote to back Breed’s re-election this November, it also voted to endorse Breed’s hand-picked replacement on the Board of Supervisors, Vallie Brown, over Democratic socialist Dean Preston.

Preston’s campaign manager, Jen Snyder, reacted by flipping the DCCC members the bird.

“That was me, an uppity, outspoken Democratic socialist who shows her mind when the DCCC endorses a candidate backed by the real estate industry,” Snyder said.

Former prosecutor, Suzy Loftus, Breed’s pick in the district attorney’s race, also got the nod from the panel.

Park it: San Francisco Recreation and Park Department General Manager Phil Ginsburg has been tapped by Gov. Gavin Newsom for a seat on the California State Park and Recreation Commission.

Ginsburg has been general manager of Rec and Park since 2009. He also served as chief of staff for a spell when Newsom was mayor.

During Ginsburg’s tenure, San Francisco has raised more than $150 million in private donations to help fund scholarships, renovate soccer fields, playgrounds and parks, and the Golden Gate Park Tennis Center

He also has been a key player in bringing big concerts, like the Outside Lands music festival, to San Francisco’s Golden Gate Park and Paul McCartney to Candlestick Park for its final event. But Ginsburg said he had no plans to push for big festivals at state parks.

When asked if Newsom had given him any mandate, Ginsburs said “yes, to show up at the meetings.”

The job pays $100 a meeting.

San Francisco Chronicle columnist Phil Matier appears Sundays and Wednesdays. Matier can be seen on the KPIX-TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call 415-777-8815, or email pmatier@sfchronicle.com. Twitter: @philmatier