The builder cannot make any changes to the original plan after the builder buyer agreement has been signed.
Once the agreement has been signed, prior consent of the buyers has to be taken for changing the original plan of the building/society.
You can file a complaint against the Builder in consumer Court depending upon the value of the case instituted for deficiency in services and unfair trade practices adopted by the Builder claiming complete refund interest as well as compensation for his actions.
Alternatively, you can also file a complaint under RERA against the builder.
Feel free to call.