Clarification-needed on service tax levied on flat purchase

Hi having been booked a flat (under construction and position on Apr2011) on 16 Feb 2010 and made 15% down payment to builder, Due some resons registration was not possible upto 31 April. But now registration stated and builder demanding the amount 3.58% of agreement value which is service tax and needs to be recovered from purchaser. Is this amount really ...payable by me to builder or not?

Your reply in this regard is highly appriciated.