Annai Builders
Changed the building plan after putting agreement

I've booked flat in annai builders on innagural day itself and within a month i've signed agreement with the builder by paying 20% amount from total cost. Now they are telling they have changed the plan and resubmitted for approval and area size will vary based on their new plan and i have to pay the varience amount or else asking me to switch to some other block or to cancel my agreement itself. I've booked five months back and I am paying interest for the amount which i've paid to builders till now. But they are telling now like this. Could some one help me how to deal this matter.