GST For Promoters - Eligible For Composition Scheme

I am doing individual house comstruction in the name of Kannan Promoters in Chennai i have taken GST in 1.7.2017 andf filed GSt Returns upto Feb 2019 in March i have received a cheque of Rs.3500000/- Including GST 12% if i file march 2019 GSTR -3b and i can opt for composition scheme for 1% or 5% in GST and i want to know the what is the GST rate for Individual House ( 12% or 18% ) Individual Houses Below 1000 Sq.Ft and 1500 Sq.Ft