Hi,
Please note that reduced rate of 8% were made applicable w.e.f. 25.01.2018 only. Any transaction done prior to the same shall not be covered in the same as that is not explicitly mentioned in the notification.
Further, if builder has already charged GST on your amount @12% and paid the same to the Govt. before 25.01.2018, there are no provisions prescribed for refund of that amount to the builder till date. As such, it is unlikely that builder will pass on the benefit to you till the time there is certainty of any refund from the Govt. to the builder.
There is an option available to you. If the builder has collected the amount from you but has not yet raised any invoice on you or not paid the amount to Govt. (which is very unlikely), he may like to transfer the benefit to you.
However, any amount taken by him after 25.01.2018, he will compulsorily have to pass on the reduced GST benefit to you, else, you can complaint against him with the local jurisdictional commissioner.
Regards,