A. Is there any penalty clause mentioned in the Sale Agreement or terms and conditions of the application form?
B. If no, you can issue a legal notice to the builder to pay the compensation for the delay of delivery of possession and you can lodge the complaint before the local authority against the builder. So the builder has committed mistakes and should bear all the damages under the deficiency in the service. And claim separate damages for mental agony.
C. File a complaint before the consumer forum to give direction to the builder to complete the construction in time to deliver the flat or claim damages and compensation for the delay of delivery of possession by submitting all material records and you can claim for refund the amount with an interest.