Late payment charges without demand letter.

We purchased one flat in month of February 2017 and paid 10% within due date .. we had to pay 40% but approval was under process with Ghaziabad development Authority. Builder finally raised the demand for 30% in month of January 2019. We paid that on time. After that in month of may 2019 we upgraded our flat from 2 to 3 BHK. Builder charged new rate on extra purchased area and we agreed to that. New agreement was signed and with that we paid second demand of 30% also on time but now with the final demand of remaining 30% builder is asking late payment charges on the first 30% which was paid in January 2019 saying that you were supposed to pay. We had funds and we were waiting for Demand letter for the same Builder is registered under RERA Now questions are 1) if approval were pending at builders side then is he still have rights for asking funds. 2) if builder has not asked any demand , so are we still liable to pay our own without any demand letter. 3) if new agreement happened with the builder and he sold extra area at new rate of 2019. Why he has not intimated for late payment charges that time if any were to be paid . Builder has never intimated for late payment s in between . Can builder do that . 4) please suggest wayout of it as we have asked builder to correct the demand without delay payment charges . 5) Can we complaint under RERA if builder Denise . How UPRERA will protect buyer in this case. Kindly suggest