I don't know about the legality of it, but the first thing I would do is find out if he can he hear every word that YOU say while he's in his apartment.

If he can, then I think a good argument can be made that your right to privacy is being violated solely by actions that your neighbor took - not you.

If he doesn't care that you hear everything he says, then that's fine. But, if he can hear everything you say and you don't want that, then he caused the problem and must fix it.

I would notify the Board if you have not already done so. The co-op's attorney may need to be consulted about this matter, too.

Good luck.

Agree with Marty. The situation you are describing, having raw cooking fumes venting into a habitable space, is an unsafe condition and almost assuredly a violation of Dept of Building regulations and code. You don't have liability in this situation, but your board needs to get involved. They need to review the alteration agreement to see if this venting was called out in the plans and part of the alteration approval. If it was, it does not absolve your upstairs neighbor from responsibility for complying with code, but it will probably cause your board to be more involved in this issue.

It it wasn't part of the plan, and the contractor just built it this way because it was convenient, then your neighbor and his contractor have a big additional renovation ahead. The configuration you described cannot remain because of the health and safety issues. Fumes from a kitchen can contain Carbon Monoxide, which is deadly.

Bottom line, IMHO, is that you are ok (just don't agree to anything, like "are you ok with the configuration?"), and your upstairs neighbor and their contractor are in deep dodo.

if two apartments are combined, DOB should have been onsite to check - you should be able to view that on the DOB site. and the plans should have been vetted by the co-op architect. this is a violation and needs to be fixed at the expense of other owner.