Cheating by Builder after allotment of flats

Hello Advocate, We booked an one BHK flat from a local builder & we got the possession too. But, now he now creating problems to us as : There was a land of 300 SQ Yards, for which builder made a contract with the owner of that land to construct the flats. So, at the time of booking builder told us (total 10 owners of flats) that he is going to construct the flats in 150SQ yards on the back side only out of 300 SQ Yards. So he left with remaining 150 SQ Yards in front portion & advised that out of that 150 SQ Yards (Front Portion ), 75 SQ Yards (Left Side) will be used by himself to build further flats in near future, we were all agreed on this term very well. Now, rest on 75 SQ (right side), he made the cemented passage & entrance to the building with a big gate. He never ever revealed us that he will cover the entrance in future to construct move flats over it by putting pillars. Now, we are in this building for past 4 months, and he trying to construct further flats over that passage. Since, we are not aware what contract he made with the owner of that land, but he never revealed this things to us prior to booking and always kept saying that this is the passage which will be commonly used to enter into the building. He told us that within 02 months, he will cover the passage and construct more flats. If he will construct the flats the ventilation, light and air would not reach to our flats as we are on the back portion of that building. This clause also not being mentioned anywhere in our General Power of Attorney too. Kind suggest how to avoid this situation. Kind Regards, Vijay Singh