A pending deal to sell almost half of Park Tower, one of San Francisco’s tallest office buildings, could lead to changing hands and set the new skyscraper’s value at more than a billion dollars — an almost unprecedented sum.

But the city will get nothing from the deal, at least in the form of the transfer taxes it collects on most sales of real estate.

That’s because owners MetLife, John Buck Co. and Golub plan to sell a 49% stake in the property at 250 Howard St., where the office space is fully leased to Facebook.

Since less than half of the property is trading, the deal won’t count as a change in ownership, exempting it from San Francisco’s 3% transfer tax on deals over $25 million, said Jeffry Bernstein, a partner and tax expert at law firm Coblentz Patch Duffy & Bass. If it was subject to the tax, the city would receive more than $16 million.

The deal would value the Transbay district building, completed last year, at $1.1 billion, or $1,445 per square foot, Owen Thomas, CEO of Boston Properties, said on a company’s earnings call this week. That would make it San Francisco’s second-most valuable building, behind only Salesforce Tower.

Boston Properties isn’t involved in the Park Tower deal, but it is the city’s largest office owner. The company recently bought a 5% ownership stake in Salesforce Tower for $187 million from development firm Hines.

The likely buyers for the Park Tower stake are Hong Kong Monetary Authority, which manages Hong Kong’s sovereign wealth fund, and Hines, according to three people with knowledge of the impending deal who weren’t authorized to speak publicly.

Property data firm Real Capital Analytics also lists the authority and Hines as the pending buyers in its database.

“We do not comment on the details of our investment activities,” the Hong Kong Monetary Authority said. Hines and MetLife declined to comment.

Thomas of Boston Properties didn’t identify the buyers. But “it’s fully leased and is being sold to a developer fund manager backed by a sovereign wealth fund,” he said on the earnings call. His description matches the combination of the authority and Hines.

San Francisco’s office rents have risen to over $80 per square foot annually, the most expensive in the country, making cashing out through the sale of minority ownership stakes increasingly attractive. Investors have purchased minority ownership stakes in some of the city’s most valuable office buildings, occupied by tech companies such as Google, Slack, Twitter and Uber.

New York, the second-most expensive U.S. office market, is also seeing an increase in partial ownership sales, which aren’t subject to a transfer tax there, either.

Partial ownership is attractive to passive investors who want income from a building, not the headaches of managing it.

“You have no say in the property. You don’t care,” said Bernstein, the tax attorney. “These become equivalent to a bond investment for a Chinese investor.”

Last year, Facebook leased all 755,900 square feet of office space in Park Tower in the largest single lease in San Francisco history. Having a blue-chip tenant like Facebook makes the building attractive to institutional investors, Bernstein said.

It was not immediately clear whether Facebook already occupies the Park Tower offices.

San Francisco voters increased the transfer tax on property sales over $5 million in a 2016 ballot measure, in an effort to make City College free for residents. While voters can change transfer tax rates, Bernstein said any efforts to make sales of partial ownership stakes subject to the tax would “have to come out of Sacramento.”

“I don’t see how (San Francisco) could amend their transfer tax ordinance. It would be contrary to the property tax rules,” he said.

The Park Tower deal may be subject to federal and state capital gains taxes, Bernstein said, and the owners will pay property tax. In another potential savings, a sale of a 49% stake does not trigger reassessment of the property, though because it is new construction, Park Tower’s assessed value will likely rise.

The Hong Kong authority and Hines previously partnered with Singapore’s sovereign wealth fund to buy 101 California St., another major office tower.

Hines has been an active developer elsewhere in the Transbay district. Along with its early involvement in Salesforce Tower, Hines also built the 33 Tehama residential building and is developing 564 Howard St., the last major Transbay tower to be built.

Roland Li is a Chronicle staff writer. Email: roland.li@sfchronicle.com Twitter: @rolandlisf