GST in Maintenance charges with regard to paid to a) if builder do the maintenance 2) if paid to RW

The residential complex is of 1286 dwelling units on a 20 acre land. It has got Sewarage Water Treatment Plant, Water Treatment Plant and a multipurpose Club House apart from the 1286 dwelling units. 
We have a TN Societies Act Registered Owners Welfare Association where in nearly 600+ owners as members. 
Still 100+ dwelling units are u see construction. 
The common properties are not yet handed over by the builder. After one year free maintenance, last year the builder did the maintenance with the help of the Association (who supports builder for reasons best known to the Association office bearers ) at The. 3.5 per sq ft which included GST.
Now people have revolted against the management committee and I'm the GBM the move to go to the builder again was rejected by majority of members. 
But Association this time also, inspite of GBMs rejection went and signed the agreement. 
Since except the management committee all owners joined together, met the builder to try reduce the rate per sq ft. He told his inability but since the rate quoted ia 3.5 including GST(same as last year)  he said if proof is given to him that GST is not applicable then, he will remove the 18% GST component. 
The complex max dwelling unit area is 1600 sq ft and so it is less than  the exempted amount of Rs7500 per month per owner. 
The present conditions are:
1. Builder has not left the complex. 
2. Builder not handed over the common properties yet. 
3. So ft area Max is 166 sq ft
4 Rate quoted is Rs. 3.5 per sq ft including GST ( 2.9+18%GST)
5. Invoices are sent to each owner by the builder in monthly basis .
6. The welfare Association just acts as a collection agent for builder and collects the maintenance cheques and hands over to builder. 
So my question is

1.As per GST law prevailing are we exempted from GST individually ,if our maintenance charges is less than Rs. 7500/-per dwelling unit. 
Kindly need the Sections and Clauses of law which exempts to produce to builder to tell him to charge us only 2.96 per sq ft and not 3.5 per sq ft including GST. 

Thanks