Gst on under construction apartments

Dear Sir ,

I have a query on underconstruction apartment flat.

1.The current applicable rate of gst

2.Gst is applicable on sale deed value

We have purchased a flat for Rs.50l

And made agreement of sale for Rs.35l

Balance 15l as other construction cost and executed bond as works contract 

State registration is on sale deed value, my question whether gst is applicable on registration value or on total value of sale

Thanks in advance for your valuable clarification