Dear Client,
If you are buying an under-construction property, it is considered as availing a service from the builder; hence, you are supposed to pay GST on it. According to the GST law, construction of a building, complex or a part of it, intended for sale to a buyer, attracts GST on the sale price.
The property that you have bought has been under-construction since 2015 and will get completed in 2018, then the amount paid or invoice raised before 1 July 2017 should have attracted a service tax at the rate of 4.5%. But any payment made towards purchase of the house or property after 1 July 2017 will attract GST at the rate of 12%.