Registration - Goods and Services Tax - GST

A builder is into construction of flats. And decided to sell flats after obtaining of CC only and also not sold nor received any money before CC. Sale after CC is covered under Schedule III hence not liable for GST. However builder has not obtained GST registration as he does not have aggregate turnover. Is he liable for GST registration u/s. 24 and also liable to pay RCM under GST for inward supplies in case the 80:20 ratio is not meet?. Thank you.