GST building maintenance cost and society formation

Hello, I purchased flat in ghansoli navi mumbai in Feb 2016 and upto april 2017 have paid 95 % agreement value with 100 %VAT and 100% service tax on total agreement value. This building is conpleted in may 2017 as per architect letter to get OC. Occupation certificate OC is recived on 30-july-2017. 1. Builder is asking for 12% GST on remaining 5% agreement value and insured that 5% service tax and VAT amount will be refunded. This property is completed before july 2017 so even though 12%GST need to pay on this remaining 5% amount?? Builder is not passed any GST benefits to us also such as input tax credit. 2. Maintenance cost is 1920 rupees per month. But 18% GST is applied on this amount. I have read in news, if per month maintenance cost is less than 5000 rupees then GST is not applicable. But builder claims that this amount collected by builder or trustee not by society. Hence GST of 18% is applicable. 3. For society formation chrges, has applied 18% GST. is this correct?? Requet to expertise to advice the GST applicability and how much % GST in each above case.