Can builder charge interest on delayed payments

I had purchased a property and as per booking form it's showing a different flat and as per agreement it's a different flat. Builder gave us an option to pay emi with pdc. We paid all the amounts due however the last 8 cheques for around 216000/- total was not paid as pdc was exhausted. Builder sent an email twice stating the amount for payment but we wanted to meet the builder regarding parking and since builder was refusing to meet we stopped the payments from 2014 onwards. On Oct 2015 he sent final reminder without sending any demand except on email asking to make payment and interest but he did not mention any amount. We again went to the office to talk to the builder but again he refused to meet us. He sent us a whatsapp recently asking to pay 21% interest on the amount payable along with interest on possession as well as society charges and tax. We have paid all the amount due on time before. However due to the builder not meeting us we did not make the rest of payment. It's approx 259000/- due. 1) Can builder charge interest on amount payable if it's mentioned in the agreement but he has not sent any formal letter asking for payment. 2) can builder send termination letter for the said flat.