Applicability of Gst on construction contract

We had acquired a flat in the month of oct 2017, for which the builder collected gst amounting to 3 lakhs. Saying that it amounts to works contract. Upon studying the act in schedule 2 of supply it is found out that gst is not leviable on the construction of a completed flat. If this would be the real intent of the act, whether can I claim gst from my buyer. Further the facts of the case are as follows. Date of completion of building is in December 2016 and of registration as aforesaid. kindly advice me.