Pent house -terrace encroachment

Hello sir, I bought a flat in a 12 flat building in 2014. There was no pent house at the time when I bought. Later it was built by builder and sold. The pent house owner initially grilled and locked the entrance of terrace stating its his private place and anyone has work with water tank, dish tv needs to take permission from him to enter the tearrace. Now he has started building sheds and encroaching the open area of terrace. I wanted to know is it legal if he has the terrace area mentioned in his registration. There is major drawback in this issue .. the building which we live is not regularised by govt of Telangana. And part of it is on a master plan of road widening and some or other day we will be asked to do something to the buidling on the master plan area. When I was buying the apartment it was the need of hour for me so i did not check the legal work.. i was not at all informed about the road widening plan when I bought. There is a no or very less chance that the building will be legalised by the govt of Telangana.(this is secondary) My query is do we have the right to go to the doors of law against the illegal encroachment of terrace by the pent house owner.. kindly reply.