“It has become a trap for the unwary,” said Mitchell Posilkin, general counsel at the Rent Stabilization Association, a group that represents 25,000 landlords. “Even the most sophisticated owners who we deal with are really perplexed about how they’re supposed to implement many of these provisions.”

The changes, propelled by the Democratic takeover of both houses of the state Legislature, sought to stem the loss of regulated apartments in New York City and to address inequality driven by rising housing costs .

They eliminated vacancy decontrol, which had allowed landlords to take apartments out of rent regulation when the rent rose above a certain threshold. They limited security deposits to one month's rent and gave tenants in eviction proceedings more time to hire a lawyer and pay overdue rent.

The legislation was a humiliating defeat for the real estate industry, one of the most influential lobbies in Albany. Industry leaders warned the changes would devastate the housing stock, and two groups recently filed a challenge to the laws.

Now many landlords are perplexed about whether they can even charge tenants who are pet owners an additional security deposit. Owners of co-op apartments are concerned about changes that were aimed at rental buildings, but could affect co-op boards and have expensive consequences for co-op tenants.

For example, the new law states that landlords can no longer collect fees to cover the “processing, review or acceptance of an application.” They are also barred from charging more than $20 to cover background and credit checks of potential tenants.

But the law doesn’t specify whether brokers, who often act on behalf of landlords, are also bound by those new rules. So many brokers have continued to charge application fees, which are often nonrefundable and, along with broker fees can be expensive hurdles to securing housing in the city.