Gst on villas construction

Sir, we wish construct 100 individual houses,  each for single family, 

Land is owned by proprietor and his wife, i wish to know whether advances recived are taxable. 

Any devlopment agreement is required to lessen GST TAX? 

Each single house is sold at 99 laks, 1 crore, and above

can we pay tax after invioce issue? 

What is tax rate, for this, 

Pls, suggest proper tax planning.