GST Implication- Query

Juhi (Student) (34 Points)

04 August 2017  
I want to inquire about GST implications regarding a situation i am dealing with.

I have given the full amount to Lucknow Development Authority, Lucknow via Demand draft on 03.05.2017 for the booking of Community Center (marriage lawn n 4 rooms attached with it) for the wedding which is to be held on 04th December 2017. Service Tax charged @ 15% on the rental value.

Now they are asking for submission of another 3% of the increased rate of GST (15%+3%=18%).
Kindly assure me whether i am liable to pay the Tax?

Kindly response n solve my query ASAP.