For many Long Islanders, basement apartments offer one of the few remaining affordable housing options. But whether those rentals are legal depends entirely on where they are located.

A search of Facebook Marketplace reveals dozens of basement apartments advertised across Long Island, with some listings promoted as “lower-level luxury suites” for more than $2,500 a month. However, some of those apartments may not be legal under local zoning laws.

Legal basement apartments are permitted in the towns of Hempstead, Brookhaven, Islip, Shelter Island and Southampton, provided they meet local code requirements and have the proper permits.

Several other towns — including Oyster Bay, North Hempstead, Babylon, East Hampton, Huntington, Riverhead, Smithtown and Southold — do not allow new legal basement apartments.

“You can’t get anything for under $2,900 on Long Island,” said Bayville resident Pamela Soss. “And in fact, I’m having that trouble now.”

Ian Wilder, executive director of Long Island Housing Services, said the region’s housing shortage is driving people toward illegal rentals.

“If our town and village government don’t allow enough housing to be built, legal, safe housing, the market will provide illegal and often unsafe housing,” Wilder said.

Housing advocates stress that affordability should not come at the expense of safety.

In March, a fire inside a basement apartment in Hempstead killed a father and son. Fire officials said the apartment did not have smoke detectors, highlighting the dangers that can exist in units that fail to meet safety requirements.

Before signing a lease, real estate broker Annie Holdreith recommends asking to see a certificate of occupancy and any required permits.

“An egress window usually has to be the size to allow a 200-pound firefighter with oxygen tanks through,” Holdreith said. “So if it doesn’t look that big without taking out a measuring tape, then it’s probably too small.”

Experts also recommend contacting your town’s building department to verify that a basement apartment is legally permitted before moving in.

Some Long Islanders argue local governments should consider allowing legal basement apartments more broadly to help address the region’s ongoing housing affordability crisis.

“Housing is tough, prices are ridiculous,” said Syosset resident Chet Singer. “You can go too far, I suppose, but for the most part, I think it’s not the end of the world when people are doing that.”