Is the sale of unauthorized room valid, can I purchase both?

In a multi-storied building, there is a generator room constructed as per the approved plan with a plinth area of 200 sq. ft. and 133 sq. ft. UDS, and a room of the same size built on top of it without approval 20 years ago. Since the generator was not purchased or installed, the builder sold both rooms to one of the flat owners in a single sale deed. Is the sale valid? I am the owner of another flat in the building. Can I buy both rooms together from the current owner? Property tax is being paid separately for both rooms.