In reference to your query, the simplest way of identifying whether your builder has exercised the option or not is to check the invoice issued by him post 1-Apr-2019. If the builder has filed the application, then the effective rate would be 1%/5% as applicable otherwise 8%/12%.
Should there be a scenario, wherein the builder has not raised any invoice till date from thereon, then you being a customer and well within your rights, can approach the builder seeking reply from him whether he has availed such option or not.
However, please note that this option was provided to the builder and not flat buyers. So if your builder has chosen to continue under old rates, i.e. 8%/12% as applicable, then you as a customer have no recourse, save to the extent of seeking passing on the reduction of ITC under anti profiteering provisions.
In case your builder has opted to switch to new scheme, then in that case, the builder would be charging you tax at reduced rate of 1%/5% as applicable, however he would be denied the benefit of ITC, which would somewhere increase his cost and the same would ultimately be factored in somewhere in recoveries made from you.
In a nutshell, if your builder has not availed the option of reduced rate, then you as a flat buyer have no recourse and will have to pay the taxes so charged by the builder at higher rate.
Please advise in case of any other clarification.
Thanks & Regards,
CA Aditya Dhanuka.