Builder is asking to pay extra Penalty

I have booked flat in 40:60 subvention scheme in May 2014 and have paid 40% amount as per BBA to builder . Builder have given the date of completion Dec 2015. Project got delayed and builder started giving Final Demand letter in July 2017 to flat Allottee(s). After that I have visited my flat in Oct 2017 which was incomplete. The builder have also given me demand letter and I was surprised to see he have imposed a penalty of 1.5 Lac Rs @ 18% for delay. But I denied make the final payment and asked the builder to complete the flat first. Then builder have not contacted me and I was also thinking that it will take time and returned to my Home town. The builder have got Occupancy Certificate from Noida Authority on 22nd Dec 2017. Surprised to see in Occupancy Certificate it is mentioned that "according to Survey done on 23rd Oct 2017 by authority OC have been issued" when finishing and flooring work was incomplete. Anyway now builder is asking to pay late penalty from July 2017 to till date ie 30th April 2018 which amount is 4.42Lac. As per my knowledge final demand should be made after Occupancy Certificate. Please Suggest. 1. Should I have to balance outstanding amount as per BBA and fight for Penalty amount in Consumer Court? 2. Should I have to pay noting and go to consumer count to get the correct Final Demad Letter? 3. Is GST applicable on remaining outstanding or service tax?