Builder has added extra interest amount, which he never commun before.

I had bought a flat in Lucknow in 2015 by bank loan on percentage completion payment method. The builder kept sending payment demands as per construction last 4 years. The bank used to disburse amount to builder when demand came. All of a sudden builder has sent me big extra interest amount, showing delay in demand payments for last 4 years. He never communicated to me about any such delay before. Now he is quoting 18 percentage written in agreement to put this extra late payment interest amount for last 4 years. No delay was communicated ever before. All of a sudden big interest amount for 4 years has come, now at time of final payment & registry.