1. for all the payments made by you before introduction of GST, the builder has already collected service tax and VAT on those payments
2. so you are not liable to pay difference between the GST rate of 12% and the combined rate of service tax and VAT
3. on the balance 5% which is due from you - there will be 2 scenarios:
if the builder has already raised an tax invoice requiring you to pay the balance payment alongwith service tax and VAT, then you will have to pay those taxes
if the builder has not raised the tax invoice and the balance has become due after introduction of GST, then you will have to pay 12% on the balance 5% consideration, as GST
4. Also remember to claim benefit under input tax credit. Meaning when the builder purchased goods and availed services, he must have paid taxes on the money involved for purchase of goods and supply of services. Then the builder again collects tax from his buyers. He does not deposit the entire tax which he collects from the buyers, in the government treasury. But from that tax he deducts the taxes he paid while purchasing the goods and availing services and only the balance is deposited by him in government account. So you must ask the builder to give you the benefit of input tax credit and not charge you full 12% gst on the balance consideration payable by you