GST rate and bill format for construction

From 01/04/2018 onwards there is an option to the builder to charge GST at 5% on the construction service without availing the ITC for the old projects. If the option is not availed the effective rate would be 12% ( 18% with 1/3rd land deduction) on the total value.
The rate for 20-21 depends on whether it s on going project or a new project and also if the option of 5% availed or not.

With above this can we charge 12% with claiming input credit and 5% for 20-21 as project is commenced last year and still it is under compeletion.