Have paid application money for 2 flats but construction has stop

I had booked 2 flats in a property by paying 20% application money. Thereafter the builder sent a demand note for next 20% for completion of plinth. And 20% was paid after first slab: (total 50%)all this almost 9 months ago. Thereafter the builder has not sent any demand note but the construction has now been stopped. No further construction has taken place since 9 months beyond the first slab ( it's a 12 storey building) I wanted to sign the agreement but now I am not sure that the builder will be able to complete construction till December 2017 as promised by the builder...! Since I have not signed the purchase deed yet which mentions the completion date as 2017 December, is it advisable to sign up the agreement now or seek a refund of my money. The builder says he will complete the construct by the due date but it seems impossible to me. He further agrees to pay 18% interest on the paid amount if the completion is delayed ( if I sign up the agreement.) What is my best option now???