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.

2) effective rate of GST payable on purchase of under-construction residential or commercial properties from builder involving transfer of interest in land or individual share of land to the buyer is 12% with full input tax credit (ITC).”

3) any payments after June 2019 would attract GST at prevalent rate 

1. rate is 12%

2. please claim input tax credit from builder

3. if builder does not give you benefit of input tax credit by way of reduction in sale price, then you can file a complaint with the national anti profiteering authority against builder

4. builder procures materials and services for the construction and pays GST to his suppliers. In turn the builder also has to collect GST from flat buyers. The GST collected from flat buyers has to be deposited with government. However the builder is not required to deposit the entire GST that he collected from buyers, with the government. The tax amount is reduced for the input tax already paid by builder for availing services and procuring goods from his merchants/suppliers. So the tax outgo of the builder becomes less. This benefit he has to pass on to the buyers by way of reduction in their flat purchase price. 

5. GST has to be paid with every instalment. And not at time of signing/registering sale agreement/sale deed 

You should pay GST after receiving Completion Certificate. 

wait for project to be completed .

Transfer of completed flats, or ready-to-move-in property will not involve any service part to the buyer. Hence, GST will not be applicable in such transactions. Because it is neither supply of goods nor service.

Hence, if you are buying a completed flats or ready-to-move-in property, then you may save the GST cost.

See if you make payment complete now only the applicable rate of GST that is 12 percent will apply but once there is revision of same then the new rate shall be applicable.

Assuming if rat is reduced then on remaing payment only 5 percent will be applicable.

See I won't suggest to wait and be a defaulter the builder can take undue advantage of same as I have no information that GST on said construction service will.drop. so it would be better if you register by paying complete amount.

If the GST is required by the government then the rate applicable on the date of the payment shall be paid by you 

You can wait, if your are sure that the GST will be reduced and also sure that the builder will not impose any additional penalty for the delay. 

Regards  

If you have booked the flat and paid the advanced gst of that amount will be levied at that applicable rate. On the balance payment with the effective reduced during sale deed execution then the effective rate is applicable. The gst rate has to be mentioned in the sale deed.

An under-construction property attracts GST at the rate of 18%. But it is not charged on the entire value of the property; it is charged only on two-third of the value. One-third value of the property is considered as the cost of land. According to CBEC, “effective rate of GST payable on purchase of under-construction residential or commercial properties from builder involving transfer of interest in land or individual share of land to the buyer is 12% with full input tax credit (ITC).”

In case 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%.

Thus any payments made now will attract the current rate of GST, the future rates are not yet been made effective, 

If you want to wait for another three months to pay lesser GST, you may do it but what is the guarantee because until nothing has come out in black and white, there is no confirmation on it