Consumer Redressal

I have booked a flat in a group housing society in 1998 ,at that time the projected cost of the flat was 938000 approx and the project was to handover by 2002-03 ,but due to the neglegence of the managing committee the project kept on delaying moreover by the time their demands were also rising . now as on date i have done the payment of nearly double the cost although due to the unesimated rise in cost their was the delay in certain installments . Till 2007 there was no question of the interest till 2012 then suddenly they say that they will charge 10% of the interest for the period of 2003-2007 and from 2007 onwards they will charge 20% as per their decision in general body meeting in the year 2007 . Now my question is that can they charge the interest as per the quoted rate for the above said duration or not kindly advise the current status and the further course of action as well . The name of the society is friends circle ltd v. regir#924GH south west zone .