The head of the city’s teachers union is asking state lawmakers to eliminate a set of property tax breaks to pay for reduced class sizes in elementary schools.

United Federation of Teachers President Michael Mulgrew said Tuesday that 90,000 condominiums and co-ops owned by people who do not reside in the city should no longer qualify for tax breaks. That change would generate $900 million each year and help pay for the 7,000 to 8,000 additional teachers necessary to significantly reduce class sizes.

It’s a new tack for the UFT, which has long fought for class-size reductions. Its newest plan comes as the growing class sizes remain a problem in New York City, with overcrowded classrooms on the rise and early grade classroom sizes increasing for a sixth straight year. Mulgrew framed his proposal as a logical continuation of Mayor Bill de Blasio’s expansion of pre-kindergarten, allowing students to receive more personal attention in early grades.

“What I would like to do now is not only cement those gains, but to amplify those gains,” Mulgrew said.

Under the UFT’s proposal, the number of students in each classroom from kindergarten through third grade would be capped at 15. The UFT contract currently limits kindergarten classes to 25 students and first through sixth grade classes to 32 students.

It’s unclear whether Mulgrew’s plan will be able to gain significant support in the legislature, which will go into session in early January.

De Blasio struck out with state lawmakers when he asked for tax increases to pay for his pre-K expansion. The legislature ultimately carved out funding from the state budget.

Mulgrew also faces a Republican-controlled state Senate, which may be less than inclined to raise property taxes on a small but wealthy segment. Union officials said they were confident that state lawmakers would be receptive, especially since many of the condominium and co-op owners affected by the tax hike aren’t New York voters.

Meanwhile, the mayor’s office responded cautiously to the UFT’s plan.

“We are developing our legislative agenda for the upcoming session in Albany. We look forward to reviewing the report and continuing our discussions with stakeholders as we set those priorities,” the mayor’s office said in a statement Tuesday.

Classroom sizes have swelled for five years, and with it the political pressure from the UFT to reverse the trend.

The union sued the Department of Education for failing to use state money from the  Contracts for Excellence agreement to reduce class sizes. The city said the money was correctly appropriated, but class sizes continued to increase due to broader budget cuts and layoffs.

The union held rallies in 2012 and 2013 to call attention to overcrowded classrooms. They won little support from former Mayor Michael Bloomberg, who once said he would prefer to have great teachers serving more students than smaller classes with inferior teachers.

De Blasio promised to reduce class sizes while on the campaign trail. Since taking office, the mayor has proposed a $12.8 billion capital improvement plan to add enough classroom space to alleviate some overcrowding.