Hello Sir,
On the basis of the information provided by you since your agreement for Sale is after the date of OC, then you are not liable for Service Tax or VAT.
Talking about the payment made before the date of OC, the builder can show it as an Advance instead of the booking amount in his books. But many builders still ask/ demand for VAT/ Service Tax. This is something you will have to sit and discuss/ negotiate with your builder.
Trust this clarifies your query.
Please feel free to call/ get back in case of any further clarifications.
Thanking You.
Regards,
Rohit R Sharma
BCOM, ACA, LLB-GEN, CERT. FAFP