A property that once was a store was rented in Queens, on the street level (1st floor), for a church. The property is approximately 20 ft x 100th feet, and as it has a big depth it may need to have an easement accessing the street through the building next door’s backyard located on the corner. My question is, is it legal to have the easement through someone’s property in the city?

An easement is, by definition, the right to use someone else’s property. I’m not aware of any restrictions on the existence of easements in that local area, so you would have to consult the local ordinances to find out. However, in a place as densely populated as that, I would be surprised to find any such law, since easements are often used to address the issues that inevitably come up with close neighbors.