The Big Apple — with its famously expensive real estate — would be hurt by a proposal in Washington to lower the cap on mortgage- interest deductions, according to a new report.

Federal lawmakers are considering capping the amount of a mortgage eligible for interest deductions at $500,000. The figure is presently $1 million.

The change would hit New York City especially hard, since home prices are so high here, a chief economic adviser to city Comptroller John Liu told Crain’s New York Business.

In other cities around the country, middle-class homeowners would still be protected — because they could easily buy a desirable residence for under a half-million.

In many parts of the Big Apple, $500,000 buys, at best, a modest apartment.