GST rate on godown construction to railway

Hai..
good morning...

I am from food industry..
for business requirements we are in the process of constructing grains storage godown to southern railway.

as per agreement railway will provide sufficient land space for construction of godown. We have right to use the godown with 20% discount for 10 years.
property will belongs to railway.

this is original work contract. kindly let us know whether we have to pay GST @ 18% OR 12% to the builder engaged for construction of godown.