Need help with service tax

if completion certificate by authority is given after u hav purchased flat then builder will charge service tax otherwise not.....

Agreed with @ jeetendra

 In your case, the issuance date of occupancy certificate by authority is important.

 If you have purchased flats from builder after occupancy certificate issued by the authority to the builder, then there is no concept   of levying service tax.

 But, if you purchased the flats before the date of occupancy certificate issued, then the service tax is leviable on you by the builder