Issue with Society

I purchased a flat in one of the multi storey buildings in hyderabad. The construction of the apartment is such that all the north facing flats are having a good corridor which no can use apart from the flat owner. Though the corridor is the part of common area , but structurally the flats are designed in such a way that no one can use that are except the owner. While selling the flats , the builder only gave the idea of having the grill if required in that corridor as no one else can use it. We are not the only family. there are about 20 north facing flats having this issue. Currently all the owners of north facing flats have putted a grill. No one objected when this activity was going on. Building association also didnot posted any issue. Now they are asking us to remove the grills otherwise they will file a case against us. Please suggest what to do and whose fault is here? while purchasing a flat are we supposed to know the society law also? If the builder themself have advised us then what can we do. Putting up of grill have costed around 10 - 11k per family. So removing the whole structure is a financial loss. who should bear this loss ? please suggest