FSI and RR payment for attached terrace.

Our plot of land was sold to the builder in 2007 and consequently a 5 storey building was erected there. As per the sale agreement we were allotted a flat in return. The builder allocated a terrace flat for us and only we can access the terrace as it has access 'only' from our bedroom. There is no mention of terrace flat in the agreement of sale. However, the builder never got the occupancy certificate from the authorities and after the recent housing society was formed they approached the builder's son (who has taken over the business after the deceased builder) and he promised help without any financial obligation. We are being asked to pay a huge amount as RR fee and premium. Are we liable to pay this as this would have come under FSI and particularly as there is no mention of this in the agreement. There is also no clause to say that the builder has sold it to us and in our opinion he allocated that flat to us as goodwill. The society says that since we have exclusive access to the flat we have to bear the cost of regularization towards FSI/fungible FSI and occupancy certificate. If we do agree to pay do we become 'owners' of this terrace ? Please advise