Despite Mayor Michael Bloomberg's opposition, the real estate industry is continuing to push for tax relief for developers who set aside apartments for low-income tenants under a incentive program known as 421-a.

The 421-a program provides developers with 20 years of property tax abatements in exchange for setting aside 20 percent of their apartments for low-income tenants.

Mike Slattery of the Real Estate Board of New York said many developers who built in the 1990s are now close to seeing those tax breaks run out and without additional tax relief, owners will want to opt out of the program and hike their rents.

"We just can't simply look at the cost of the taxes that are foregone," he said. "We have to balance that cost with what it would cost to replace these units."

The mayor has said the city needs the property tax revenue to pay for services.

Brad Lander, a Brooklyn City Councilman and former housing advocate, argued that developers can't opt out of the 421-a program for another 10 years.

Slattery said developers are seeking to cap property taxes at 20 percent of their gross income and right now property taxes are at 33 percent of gross income.

State Assemblyman Vito Lopez, chair of the Housing Committee, said the tax break would cost half a billion dollars over three years.

Slattery's group estimate the cost at $40 million over three years for 25,000 apartments — 5,000 are considered low income units.