Refer Sl.No 5 of Schedule II to the CGST Act:
5. Supply of services
The following shall be treated as supply of services, namely:-
(a) renting of immovable property;
(b) construction of a complex, building, civil structure or a part thereof, including a complex or building intended for sale to a buyer, wholly or partly, except where the entire consideration has been received after issuance of completion certificate, where required, by the competent authority or after its first occupation, whichever is earlier.
Explanation.-For the purposes of this clause-
(1) the expression "competent authority" means the Government or any authority authorised to issue completion certificate under any law for the time being in force and in case of non-requirement of such certificate from such authority, from any of the following, namely:-
(i) an architect registered with the Council of Architecture constituted under the Architects Act, 1972; (20 of 1972.) or
(ii) a chartered engineer registered with the Institution of Engineers (India); or
(iii) a licensed surveyor of the respective local body of the city or town or village or development or planning authority;
(2) the expression "construction" includes additions, alterations, replacements or remodelling of any existing civil structure
In view of the above provision, no tax liability arises if the flats are sold after the receipt of the OC/CC. However you need to pay tax on inwards supplies of goods & services including under RCM as applicable. There is no exemption of tax on inward supplies [purchases] if outward supplies are not taxable.