When media executive Lachlan Murdoch paid $150 million for a mansion in posh Bel Air in December, a record price for Los Angeles, he did so presumably knowing he would owe over $1.3 million a year in property taxes, public records show.



It’s an effective tax rate of around 0.9%—about the same as a fellow record-breaker in distant Palm Beach, Florida, where that same month, hedge funder Steven Schonfeld snapped up a megamansion for $111 million, taking on a property tax liability of around $1 million, public records show.



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The two megamansions stand in stark contrast to New York City’s blockbuster $238 million condo sale last year—the most expensive home deal ever in the U.S.—for when the owner, hedge fund manager Kenneth Griffin, settles his $531,797 tax bill this year, he’ll pay an effective tax rate of only 0.22%.



That rate is lower than Laredo, Texas, the cheapest property tax jurisdiction in the country, according to a report from ATTOM Data Solutions. More importantly, the billionaire pays a fraction of what working-class homeowners in the neighboring Bronx pay, relative to their home values, according to a city commission that’s calling for a major overhaul to the city’s convoluted property tax law.



“There can be vast differences in how properties are classified, valued and assessed, lending credence to the widely held characterization that the system is overly complex, opaque and arcane,” commissioners wrote in their preliminary recommendations published at the end of January.



The changes would tax co-ops, condos and one- to- three-family homes in a unified way and do away with certain tax caps, which would in effect raise property taxes on high-end real estate in affluent areas while lessening the tax burden on lower-income homeowners. They would also bring the city in line with standard practice elsewhere, such as Los Angeles and Palm Beach counties, for example, where all homes are assessed and taxed at full-market value.



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One of the quirks in the current antiquated system makes some co-ops and condos, assessed using comparable rental buildings, vastly undervalued for tax purposes. That means uber-luxe condos currently paying tax on a fraction of their market value would see the most dramatic increase in property taxes under the proposed changes.



Mr. Griffin would see his tax bill grow more than fivefold to nearly $3 million a year, according to a hypothetical example in the commission’s report.



Using that same calculation, Mansion Global looked at the top 10 most expensive condo sales last year, and how their tax bills would rise under the proposed changes.



Among them, some trophy homes along Billionaires’ Row in Midtown Manhattan—an already softening segment of the city—would see tax bills more than quadruple. That includes the home of music star Sting, who snapped up a penthouse in the same tower as Mr. Griffin last year for $65.75 million. He would see his tax bill grow 489% from roughly $140,000 to a little over $825,000.



Further downtown, Amazon CEO Jeff Bezos, who bought the penthouse and two neighboring condos at luxury development 212 Fifth Avenue for $81 million last year, would owe the city over $1 million, more than doubling his current tax bill.



“It will wreak havoc,” said Douglas Elliman agent Holly Parker. “It would just cause absolute pandemonium. You can’t change people’s obligations that dramatically, even with a phase-in period.”



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Manhattan’s luxury segment has slowed considerably over the past two years, after federal tax reform in effect eliminated state and local tax deductions for high-income jurisdictions such as New York City and limited the amount of mortgage interest homeowners could write off. The market is also digesting a hike in the mansion tax that went into effect in July.



Ms. Parker said two of her biggest clients, wealthy families from Thailand and the Middle East, have decided to stop doing business in the city because of an array of new regulatory and tax changes. She predicted a spike in property taxes would only drive other affluent investors out of the city.



“If you’re sitting in the middle of an empty movie theater, do you then raise the price of a ticket?” she said.



Of course, nothing will happen overnight. The commission will hold public hearings on the proposals over the next few months before issuing a final report, which would have to win approval from Mayor Bill de Blasio and the City Council before heading to the State Legislature and Gov. Andrew Cuomo. At the moment, the proposals are still missing key details, including what the tax rates would be, the length of the transition period for existing homeowners before they see the full effects of the change, and various provisions and abatements for primary homes and low-income residents.



At the moment, the current proposals don’t do enough to address the overwhelming number of households—of all income levels—that bought property in pockets that have undergone intense price appreciation and gentrification, said Donna Olshan, president of Olshan Realty.



“There are many, many New Yorkers whose net worth is tied up mostly in their real estate, and if their property were assessed at market value, it would be like serving them an eviction notice,” she said. Such areas include highly gentrified parts of Brooklyn, such as Prospect Park and Cobble Hill, as well as co-op buildings on the Upper West Side and Upper East Side, where property taxes have remained relatively low.



“You have an older population of middle- and upper-class people whose homes are now worth maybe a couple of million dollars. If you want to create a hornet’s nest, you just start with their property taxes,” said Ms. Olshan, adding that the proposals would create such a political minefield she doubted they’d be adopted.



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Frederick Peters, CEO of Warburg Realty, said the motivation to create a more equitable and transparent system was noble, but feared its implementation would be haphazard and would certainly affect the amount people are willing to spend on New York City real estate.



“If the taxes are much higher, then the monthly mortgage has to be lower. If the mortgage has to be lower, than the price will have to be lower. There has to be some reversion to the mean,” Mr. Peters said.



Property taxes alone are unlikely to drive someone to sell their home, Mr. Peters added, except “if it feels to people like the straw that breaks the camel’s back, because they’re already paying more in city, state and local income taxes and an extremely high capital gains tax, and they could move to Florida where much of that is mitigated.”