GST on Property/Apartment Redevelopment - URGENT

Hello Everyone, 
I wanted clarity on GST levied on Apartment redevelopment. My apartment has 4 apartments which the builders are planning to rebuild to 6. Builders gave me a GST calculation where they use Super built up area multiplied with new apartment sale value in the area on which 5% GST is levied. They do reduce 1/3rd land area, but also cite recent Gujarat High court judgement on reducing total land area while computing the amount on which GST is to be levied. Please do help me identify how GST is calculated for apartment redevelopment? This is for my apartment and not the 2 new apartments to be constructed.