Builder demanding extra money for change in architecture plan

Hello, I recently received a call from the builder with whom I have invested in a 2 BHK property in mumbai. The property is still under construction and the possession is in 2 years time. I have already completed the registration formality of the property and it specifies clearly the apartment specs and built up + carpet area etc. All of a sudden I am now being told that the builder has decided to add balconies to all forward facing flats in all the wings. This was not the initial architectural plan the property was sold on. Rightly so, they want the customers to pay extra money for the same. I find this unethical as this wasnt the original plan. I feel the builder is trying to make extra money by using this sales strategy because all buyers now have no option but to pay extra. For middle class people like me who plan their finances this is an added cost which wasnt sold to us in the first place. Please can you advice what my rights are here? The builder says that if majority people give consent to this then the others will have no choice but to pay. Please advice.