I think in the builders agreement he claimed delay interest charges and if any late payment possession delay charges will not applicable.
Further it is very much frustrating to me that suddenly an Interest amount is appearing in your Last Demand Letter whereas in all earlier Demand Letters there is no mention of such Interest.
For instance, in the last demand letter sent by you , it does not show any interest changes. However, looking at the amount of Interest and as per discussion with you, you are charging one time interest for many of the past different payments made by me at different dates. This is not as per prudent accounting norms to charge interest immediately in next demand letter instead of waiting till Final Demand.
3. Further , I would also like to bring into your notice that there is considerable delay in the possession of my flat because I received the last intimation from you that you will be giving the possession in December 2014 but the actual possession is being given in October 2015 . For a Customer this is also a loss which you should consider and not charge above interest in unreasonable manner.