Nearly every debate about New York’s housing crisis involves a state program called “421a.”

But what is it?

On one level, it’s a simple idea: Give developers a property tax break to build housing in New York City that might otherwise be a drag on their bottom line. The program was first put in place 50 years ago, and its proponents — including developers and politicians across the political spectrum — say it is one of the main reasons that any rental properties are built in a city with some of the most expensive housing in the world.

It’s also extremely controversial.

In 2022, the law expired, and the governor, city officials and many state lawmakers have been trying to bring it back ever since — including by introducing a new version this legislative session. But left-leaning lawmakers have fought the program’s resurrection because they say it is a giveaway for developers.

Mayor Eric Adams and many housing experts believe a new tax incentive is necessary to address the housing shortage at the root of the city’s affordability problems. Without more supply, they argue, demand for housing will continue to drive costs ever upward.

Even some of the program’s critics now acknowledge there needs to be a tax incentive that also mandates that rents on some new apartments be kept affordable. But any new law faces major hurdles, including criticism that the old one did far too little to address the city’s worsening housing crisis.

What did 421a have to do with affordability?

To receive the 421a tax break under the program, also known as Affordable New York, developers had to make a certain percentage of units “affordable” to people and families at different levels of income. Affordable in this case meant that rent should not exceed 30 percent of a household’s income.