Ask an Property Solicitor. Get an Answer ASAP.
On what basis was it built
A gift or something else
Let me see if I am help and please rate 5. His understanding of the law is wrong and even if it was correct he says he can only remove with the owners consent. The owner does not give consent. In any event the studio was a joint project. He can makes claim against you financially but not to remove the studio. Please rate 5 stars