terrace flat redevelopment

Hi, We stay in Thane city. Our flat is a terrace flat on last floor(3rd floor, terrace is open to sky). The terrace along with its area is been mentioned in the ragisterd sale agreement. In addition, it is also shown in the approved TMC plan of the building. The building was constructed in 1979. Our tax asessment also mentions the terrace and we are paying property tax accordingly. There is another terrace alongside of our private terrace which is a common terrace of the society. Our building is going for redevelopment. Builder agreed to give us the terrace. However, the proposed new plan has the terrace attched to flat but the entrance to the terrace is from common lobby. Builder's architect says he cannot give entrance through flat as it is againts DC rules (thane) and asked us to pass a resolution and take the ownership of the terrace with noc from society. However, as per my understanding currently we are rightful owners of the terrace as the entrance to the terrace is shown via bedroom in the current plan but what builder is asking us to accept seems to me is just a provision to use the terrace as a personal terrace but it will not remain rightfully ours. I would like to know is there really a rule in thane dc rule book that says flats with attached "open to sky" terrace are not allowed. Or builder is not giving us our fare share? Please guide how do we put forward our point in front of the builder? is there any leagale backing for us to demand the terrace that should be shown on approved plan or can we at least demand 1/3rd area in compensation against giving away the exiating terrace? Thanks