Buyers with big budgets are bewildered. Brokers, builders and building boards are apoplectic. It’s only been a month Mayor Zohran Mamdani’s so-called “pied-à-terre” tax on second homes valued over $5 million took effect, and the city’s real estate market is already reeling.

“The numbers are so beyond what people are interested in spending every year that many people are rethinking,” said Pamela D’Arc, a Compass agent who frequently sells luxury city residences to tech and Hollywood execs from California, told The Post. “People are listing things for sale that they don’t use a lot. Buyers are sitting on the sidelines. I have clients that have changed their price point to be just under $5 million.”

Highly priced new buildings that lure out-of-town buyers are particularly vulnerable to the tax, which starts at 4% and rises to 6.5% annually in Phase 1. (A second phase will update the calculation in the 2028/29 tax year).

“We had a buyer from Madrid who also has a home in Miami. She was very attracted to the [new] Giorgio Armani Residences at 760 Madison Avenue, partially because of the name, partially because of the location,” Douglas Wagner, Director of Brokerage Services at BOND New York, told The Post. “She had been watching a particular unit pre-market while it was still under construction, and when the price came down, she got serious. But we were coming up on July 1, and we told her about the pied-à-terre tax and what to expect. Around July 11th, she let us know that she was suspending her search because of the extra expense.”

That unit, No. 6B at the fashion-branded Lennox Hill condominium has common charges and taxes that add up to roughly $120,000 per year. Based on its city-calculated valuation, the $8.9 million unit would eventually owe another $40,000 to $50,000 in annual tax, Wagner says. That sponsor unit is still for sale as a result.

The many “empty towers” of Billionaires Row, both lauded and lampooned as Swiss bank vaults for the globetrotting elite, are likely to be hit hardest.

At 432 Park — where Jennifer Lopez and Alex Rodriguez, as well as billionaires like Saudi property sultan Fawaz Al Hokair, have inked deeds — at least one third of the units will be subject to the tax, the Wall St. Journal reported. That puts owners at the building on the hook for about $3 million in tax on top of the $3.6 million in property taxes owners already pay. Many of the units at that super-tall building are owned by hush-hush LLCs, and if they too turn out to be owned by out-of-towners (which they almost surely are) that figure could be much higher.

Another Billionaire’s Row ultra-tower, 220 Central Park South, is also on shaky ground. It’s home to musician Sting and a cache of billionaires, including prime Mamdani target Ken Griffin, who has a $238 million, practically unused, playpen.

In his kerfuffle with the mayor, Griffin has threatened to withdraw his massive fortune from NYC. No wonder: He could be on the hook for another $1 million a year, on top of the $837,000 in property tax he now pays, the Journal reported.

But, as wealthy pied-a-terre buyers change their shopping habits to limit their tax exposure, the real estate market is heating up in surprising places.

For decades, co-ops were Manhattan’s most sluggish property category — suddenly they’re hot.

“The co-op market is the value buy, as it has been for years,” said D’Arc. “Now, people who had been shopping for condos are looking at co-ops that are pied-à-terre friendly.”

The math is simple. In the first quarter of the year the average condo resale in Manhattan with three or more bedrooms (just the kind of plush pad part-time New Yorkers covet) closed for over $6.7 million, according to a Brown Harris Stevens study. The same category of co-op sold for just $3.7 million — coming in under the tax threshold.

And while many prestige co-ops shun pied-à-terre buyers, others welcome them and their share-boosting dollars.

New condos towers with amenities a la mode and inventory that skirts the tax threshold are also seeing action. At the 65-unit, 2023-built Mandarin Oriental Residences, Fifth Avenue, broker Peter Zaitzeff credits the tax for sending buyers who would have otherwise spent more his way.

In June, the building inked goose-egg deals; but since the tax kicked in Zaitzeff has moved two. Those units 22A and 19A were asking $3.95 million and $4.995 million, respectively, skating just under the number that activates the tormenting tax.

“If you look at the building makeup, 95% of the remaining inventory is under $5 million,” the Serhant broker said. “Its actually, sort of, I don’t know… benefited us. We are doing more sales now than we did prior.”

Another building that’s seeing upside to the downside is One Wall St., Harry Macklowe’s ambitious 566-unit residential conversion of an Art Deco skyscraper.

“We did a study on what level our existing population would be affected by the tax — and it was close to none,” said Anna Zarro, President of One Wall Street Sales.

In fact, Zarro expects to transact off the new tax. Virtually all 32 of the building’s current listings won’t have to pay. At the same time, it has a 100,000-square-foot smorgasbord of swank supplementaries — from a private restaurant, 75-foot pool, fitness center and spa, to  co-working spaces and speciality children’s space. Downstairs there’s the luxury department store Printemps and a Whole Foods. All of that makes it ideal as a home away from home.

“We’re extremely well-suited for those that have multiple homes, with a wide range of residences under the $5 million threshold,” she said.

Meanwhile, ultra-luxe rentals are booming.

“I’m seeing a ton of activity in the high-end rental market,” says Caroline Bass of Corcoran. “These people, maybe they would have bought. Now, they’re deciding to rent a six-figure apartment.”

Top towers like 30 Park Place, 56 Leonard and 111 Murray St. in Tribeca, all loaded with $20,000-plus-a-month rentals, are catnip to this rich, but not insensitive, demographic.

Brokers say these units are already moving. Zaitzeff notes that the penthouse at the Bellemont at 1165 Madison Ave. on the Upper East Side, was rented for $85,000 per month at the start of July.

“There are some people who just press pause,” added Zarro. “There are some people who are waiting to see how things play out. And there are some people who are renting very, very high-ticket condos. They’re doing the math on various taxes, including the pied-à-terre tax, and they’re saying, ‘Fine, I’ll rent.’”

But, while some are figuring out their way around the tax and others are finding small benefits, overall, it’s a net loss for NYC and it’s real estate market, insiders insist.

“If anything, given the dire strait of the city budget, we should be encouraging more transactions, not less — because the more velocity we have in the market, the more real property tax gets generated, and that goes right into the city’s general fund,” said Jason Haber, a Compass agent and co-founder of the American Real Estate Association. “Instead this tax does just the opposite.”