Because he can’t stop you from setting up your scaffold on his property if that’s the only way to finish your job—and you are willing to pay him. It’s a trade-off. This is New York City, after all. Sometimes—in fact lots of times— the only way to work on your building is to set up on your neighbor’s property.
Therefore, the law in New York (RPAPL § 881) says, if the only reasonable way to work on your building is to temporarily use part of your neighbor’s property, he can’t stop you. But you can’t hurt his property or take longer than necessary—and a judge can order you to pay him for his trouble, usually monthly until you’re finished. The amount will be set by the judge if you and the neighbor can’t agree. But for that, however, you get to work on your building from your neighbor’s property.
If you have problems with your real estate in New York, call Andrew Weltchek.