Terrace rights to the builder

I have purchased the second floor house from the owner of the site in a four floor building. Owner stays in the first floor. Third and fourth floor belonged to the builder out of which the third floor has been sold by the builder and he has retained the fourth floor. The builder has built an unauthorised construction on the terrace which comprises of a hall and the bathroom. The Syntex water tank is kept on its roof. We were not aware earlier that it's an unauthorised illegal construction. Recently he tried to extend that building to which We the second and third floor purchasers objected and are demanding that he should demolish this illegal construction entirely. We have complained to the BBMP also. Now the representative of the building calls and informs that the builder of the building has 20% right over the terrace and the building on the terrace is built for keeping the water tank! It has four walls, windows and a bathroom and both builder and the owner were all these days refusing to hand over the keys to us. Please let me know whether he is right in his claim. We are not ready to withdraw the complaint till he demolishes the entire unauthorised construction, keeps the terrace open to the sky. Please guide me.