Delay of more than a year in possession of flat

My builder was supposed to give our apartment by December 2011 but they kept on extending the possession and we haven't gotten possession yet. Due to that I have to take rental property and a hefty amount is going out of my pocket. The agreement doesn't state any penalty from builder's end in case of delay, however it states  20% penalty in case of delay from customer's end. The agreement states the possession date in December 2011.