GST ON REAL ESTATE

I bought a new flat in Surat, agreement was made prior to 1st july ie before gst implementation, and down payment was as well paid at that time, 95% of building work was completed at that time, loan for the same was sanctioned on 16th July,
Now the builder is demanding extra money for Gst....what are the implications....??
Need an expert advice...should we laible to pay gst or it is the duty of builder...??