Short-term leases, as often advertised on real estate websites, may seem like a good idea, but they’re actually against the law and can possibly lead to steep fees or other punishments. NY1’s Jill Urban filed the following report.

Ads for short-term leases, generally with fees, are constantly on display on Craigslist and other rental sites. They seem harmless, but most people don’t realize they’re illegal.

“Renting out your home in New York City for less that 30 days has been illegal since May, but many of the people doing it now, there are hundreds if not thousands doing it every week, don’t realize that,” says Teri Rogers, founder of BrickUnderground.com.

Changes were made to the law back in May to keep visitors safe by offering specific requirements for buildings that can be used as hotels. The net effect is that anyone in a multiple-dwelling building is prohibited from renting out a home for less than 30 days.

Rogers says many New Yorkers who rent out their homes have no idea they are breaking the law and those who do assume they won’t get caught. That’s not true.

“The law is enforced in two ways. One is complaint driven; could be your neighbors, your landlord, your under-tipped super or a doorman can call 311 and complain. There is also an interagency taskforce established by the major to go undercover to scour the popular websites and pose as a renter,” says Rogers.

Real Estate attorney Steven Wagner says most landlords and boards already had rules prohibiting short-term leases because they create health and safety concerns for other residents, but the law makes it illegal with stiff penalties.

“The fines can range from 1,600 to 5,000 on a first offense, and they go up exponentially after that. But there are a whole other set of issues with the landlord and you can lose your apartment,” says Wagner.

In extreme cases, repeat offenses could land people in jail.

There is, however, one loophole.

“There is a loophole, and if you have a lease, the law allows you to have an additional occupant as long as the landlord knows that that additional occupant is there and you reside there at the same time,” says Wagner.

In other words, a roommate is okay, but renting out the space otherwise could be opening the door to trouble.