Builder changes the superbuiltup area on project completion

I booked a appartment early 2013 with a superbuiltup area of 680 sqft. But now when the building is nearing possession, the builder says that the superbuiltup area has increased by 30 sqft. And he is asking me to pay for that area. This extra 30 sqft was not a part of the initial agreement when I booked the flat in 2013. Builder denies possession of the flat until I pay the extra amount.