Open Terrace in apartment

Sir, We have bought an apartment on the 3rd floor in an apartment. The apartment is a joint venture between the previous land owner and the builder. The builder has got two 3 bhk flats and 1 bhk flat which is close to 3300 sq ft. The previous land owner has got two 2 bhk flats and one 3 bhk flat. We have bought one 3 bhk from the builder on the 3rd floor. The land area is 2700 sq.ft. 6 flats have been built in the apartment. There is an open terrace on the 4th floor and a devotional hall has been constructed for the previous land owner. Now the previous land owner claims that the terrace and devotional hall ownership belongs to only them. We being the owner of a flat in the apartment, demanded for the terrace access and they are not at all willing to provide the terrace access claiming they have a separate understanding with the builder. This was never mentioned to us before registeration. Its not mentioned in our registration paper as well. Builder has sold all his flats and there are now 3 other owners excluding the previous owner. Is this the right law claiming the terrace ownership? Could you please suggest? Thanks, Divya