According to entry no 5(b) of Schedule II, 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.
The above is the given definition of construction services, which excludes the transaction where the entire consideration is received after issuance of the completion certificate or after its first occupation. This is why it results in the sale of immovable property after completion and which is neither the supply of goods nor the supply of services as per entry no 5 of Schedule III.
In the given case, Part OC has been received for 23 floors out of 28 floors tower, which to my understanding is not possible, as rightly pointed out by Shilpi Ji, that under RERA, it is one project and due to which Part OC as a concept I am not able to understand, however assuming it is possible -
Upto 23 floors: For unsold flats which were sold after receipt of part OC is neither supply of goods nor supply of services (outside purview of GST) as per entry 5 of Schedule III. Thus, GST is not payable on the same.
However, if a few of the flats from the 23 floors were sold before receipt of Part OC then the same supply will fall under entry no 3 of Notification no 11/2017(R), thus being a supply of services would be taxable @18% generally or any other deferred rates as per said entry.
For 24-28 floors: Even if Part OC is received, that is for up to 23 floors. Thus, 24-28 floors are still to be said as going under construction, and before receipt of OC for the said floors, if sale is made, then it will not be exempt and will be taxable as supply of services under entry no 3 of notification no 11/2017. Thus, GST would be payable on the said transaction.