Gst on under construction flat - 12% instead of 4.5%?

I recently booked an under-construction flat assuming a service tax of 4.5% over the agreed price.

I am planning to make the payment after july'17.

Considering that the GST is scheduled to be aplied from 1st July'17 and the GST rate decided on real estate is 12%, would I be liable to pay 12% over the price in place of 4.5% (A huge increase of 7.5%)? Or would the developer be able to use transitional provisions and reduce the effective GST rate aplicable?

Also, what would be the rate applicable if I were to sign the flat documents and make partial payment in June and remaining payment after July?

Would really appreciate advice on this matter. Thanks in advance.