More than 1,700 property owners could be removed from the city’s 421-a tax program as a result of failing to file a mandatory Final Certificate of Eligibility.

The New York Post reports that the city’s Department of Housing and Preservation along with the Department of Finance reached out to notify over 5,200 property owners that they were at risk of losing their exemption if the form wasn’t filed by January 5. Apparently 1,788 of those folks failed to register by the deadline, causing city officials to suspend their tax breaks. The suspension affects more than 11,000 apartments.

The 421-a program gives developers tax breaks that last for several years for including affordable housing in the multi-unit residential properties. It’s been in place since the early 1970s and while controversial, it has proven to be an effective way of increasing the city’s affordable housing stock.

City officials are giving property owners one more chance to remain in the program and will reinstate benefits to owners that respond by May 1.