When Scott Golden and his wife Stephanie Cohen sold their Upper West Side condo in July, they thought they were done with New York’s sky-high taxes.

Instead, they were slapped with an additional $48,949 bill on the way out — roughly 25 times their regular property tax bill.

The Miami-based couple, who run an insurance brokerage firm, are among the thousands who were retroactively slammed by the state’s controversial new pied-à-terre tax on luxury second homes.

And now they’re teaming up with other homeowners and a New York City co-op in the latest lawsuit contesting the legality of a new state-imposed tax that was initially pitched by Gov. Kathy Hochul and socialist Mayor Zohran Mamdani as targeting billionaires — but also hits owners of less expensive second homes.

“I think it’s blatantly unfair that they are targeting and discriminating against out-of-staters,” the 63-year-old Golden told The Post.

“And there was no ability to plan in advance. If we had known, we would have sold earlier,” added Golden, who got $6.55 million for the Riverside Boulevard property.

The suit, filed Sept. 29 against the Hochul administration in state Supreme Court, alleges the second-home tax violates the US and state constitutions by unlawfully discriminating against nonresidents, imposing unfair burdens on homeowners and cooperative buildings and applying retroactively to Jan. 5.

That’s nearly five months before it was enacted on May 28 by Democratic-controlled state Legislature and Hochul at the behest of Mamdani to raise $500 million for the city budget.

Others who’ve joined the litigation include Kent Barwick, a New York City civic legend who previously headed the city’s Landmarks Preservation Commission and the Municipal Arts Society, and his wife, June.

Barwick purchased his co-op apartment on Mott Street in Little Italy in 1972 and lived there full-time until he and his wife moved upstate to Cherry Hill a few years ago. They kept the co-op, which the city values at nearly $2.1 million, as a part-time residence.

When the second-home tax kicked in, the senior couple was slammed with a whopping $83,603 bill on top of the $24,984 they must pay in normal property taxes — bringing the entire tab to $108,587.

“For us, for people of modest means, both retired, it meant suddenly getting a property tax bill that was four times higher than what we were paying,” said Barwick, 85. “Now we have to figure out a way to come up with an additional $80,000.”

June conceded they might have to sell the co-op if they can’t come up with the money.

The plaintiffs and others being slammed have to pay the new tax by January.

Mamdani vowed to forge ahead with the tax rollout two weeks ago even after a judge ruling on a related lawsuit ordered the city to go back to the drawing board and essentially start the process from scratch. Instead, the city opted to appeal the decision.

Jim Whelan, president of the Real Estate Board of New York, which is funding the lawsuit and vehemently opposes the new tax, said the perils raised by the plaintiffs show that the “second-home tax is hurting New Yorkers, not the out-of-state billionaires its supporters promised to target.”

“Their stories demonstrate why this tax is misguided, unconstitutional, and unfair to the very people who live, work, and invest in New York,” he said. 

The Mayor’s Office did not return messages.

Hochul spokesperson Kassie White said, “While this lawsuit about the administration of the tax is a matter for the city and the courts to work through, the governor continues to believe that people who can afford a multimillion-dollar second home in New York City can afford to pay their fair share.”