Delay in possession of flat

Booked flat in Dec 2011 in Lucknow. Possession date was 31 Dec 2013. As per agreement I paid full amount before 31Dec 2013. In case of delay builder has to pay interest @15% pa. Due to delay and not building as per law, municipality withdrawal all permissions. Builder not getting Completion Certificate. Shall I approach Consumer court before registry of flat or after registry in my name. Can I sell flat after registry and still claim interest and compensation from builder?