GST payable under RCM by builder

No. The RCM is not applicable on builders flat.

Dear Mvranana
As per Notification 4/2019 : TDR rights from landowner to promotors is fully exempted to the extent of all residential apartments sold prior to Completion Certificate. So upto the extent of All unsold apartments in the project, proportionate tax would be have to be paid on RCM
That means , need to include all unsold (both promotor & landowner ) flats in regard to compute RCM liability for TDR.
Let's understand with exemple.
1. total flats 20 (100 Sq Mt each)
2. landowners owns 10
3. Promotor owns 10
4. value of 1 flat is 1 crore
unhook flat balances (i) 10 (landowner)
unhook flat balances (ii) 5 (developer)

value of development rights provided by developer to landowner will be 10 flats x 1 crore = 10 crore

10 crore x (10+5) x 100 Sq Mt. × 18% / 20(total flat) x 100 Sq.mt
= 1.35 crore
so need to pay 1.35 crore under rcm .