Regarding your query, please go through one of the explainations given under FAQs section of CBEC website:
The Sec. 142(10) and 142(11) of the CGST Act, 2017 provides for the provisions to deal with the liability towards the ongoing projects. These provisions are explained with reference to the following possible situations:-
(i) when the total consideration was received prior to 30.06.2017 from the customers in respect of the property under construction (for which neither occupancy certificate was received nor it is yet to be occupied) - Service tax is/was payable on the consideration received @ 15% on 1/4th of the consideration; and there would be no GST on the same. ( Sec. 142(11)(b)- refers);
(ii) when a part of the consideration was received prior to 30.06.2017 from the customers in respect of the property under construction (for which neither occupancy certificate was received nor it is yet to be occupied) - Service tax (ST) is/was payable on the consideration received prior to 01.07.2017 i.e.: @ 15%
on 1/4th of the consideration; and there would be no GST to the extent of that amount for which ST was paid/payable. For the remaining consideration paid/payable on or after 01.07.2017, GST is payable with reference to the date of payment of the balance amount or the date of invoice issued by the builder, whichever is earlier (generally invoice reckons to the payment milestones as per the agreement between the builder/developer and the buyer).
(iii) In respect of an ongoing construction project(for which neither occupancy certificate was received nor it is yet to be occupied), when the milestone for payment was achieved by the builder/developer, who raised an invoice within 30 days from the same(as required by law) prior to 30.06.2017, but the payment is received from the customers in respect of the said invoice on or after 01.07.2017, - Service tax is/was payable on the consideration so received @ 15% on 1/4th of the consideration; and there would be no GST to that extent. On the balance amount payable or paid w.r.t the subsequent payment milestones falling on or after 01.07.2017, GST is payable, as mentioned at (ii) above.
(iv) when the total consideration is received, as per the agreed terms, on or after 01.07.2017 from the customers in respect of a property under construction (for which neither occupancy certificate was received nor it is yet to be occupied) - GST is payable @ 18%, on 2/3rds of the consideration.