Gst for amenities is applicable in new apartments.

I have bought one apartment sft 1800 x rs 3950 plus amenites additional 4 laksh in oct 2018. Apartment will be ready by sep 2020. 1. Is it required to pay GST for amenities also . 2. Regsitration is done for 1800 sft which is over all sft. Carpet area is only around 1100 sft + balcony 150 sft . And car parking 100 sft part of amenites. So can we split the cost of carpet area and pay gst only for carpet area. 3. Builder is giving us input tax discount of 4% only. So is there anyway to ask builder to reduce the GST further.