Date of applicability of 30 % service tax portion

Dear Sir /Ma'am. I have booked an under construction flat on 01.01.2016 in Deepganga Apartments pvt Ltd Haridwar. Paid the 2nd instalment on 23.04.2016 @ 3.625 %. Now the 3Rd installment was due on 01.06.2016 and it is to be paid. But my builder is asking for the balance of service tax not charged in 2nd installment. He is saying that in 2nd installment applicable is 4.35 % because of increasing in the portion to 30 % wef 01.04.2016, from 25% previously charged. As you know that union budget 2016 the portion of under construction property is changed to 30%. (My flat was booked on 01.01.2016). I need guidance on following questions - 1. What is the applicable date of charging on 30% portion. Is it 01.04.2016 or 01.06.2016? 2. I have made payment of 2nd installment to my builder on 23.04.2016 @ 3.625 % (25% portion of 14.50 %), he is now asking for the differential payment of service tax for the not charged 5% portion on 23.04.2016. Is this demand right? 3. Do the new application service tax portion (30%) should be applied to the rest of payments to my builder, as only 25 % portion of service tax was taxable on the date of booking?... Please help. The date of applicability of 30% portion and being chargeable on previously agreed contacts are key questions for me.