Dear Rasesh,
There are could be different ways of agreements that a buyer can enter into with the Builder
1) First to buy the land and another for a works contract with the builder to construct the house. In this case, WCT may be applicable (Refer: 2008 (10) S.T.R. 357 (A.A.R.))
2) To enter into an agreement with the builder to buy an apartment, the Sale Deed of which will be executed once the construction is complete. During the period of construction, the buyer will be making the payment towards the installments to the builder towards the relevant residential unit the buyer has booked. In this case the service will be taxed as Construction of Residential Complex Service (Refer:2008 (10) S.T.R. 341 (A.A.R.))
Hope this helps.
Thanks,
Ganesh