The builder Asking interest on Demand money

Dear Sir, I have purchased a flat from the builder which is under construction . The building has four floor .Now construction is done upto 2nd floor i.e only foundation .pillar & roof work is done .The cost of flat is 34 lacs .The builder already taken 17 lacs .For every floor roof is casted he is asking 10% of the total cost of flat .The agreement I signed , it is written on it .Though he is demanding the money as per agreement , but he is charging money at higher proportion rate than the actual upto date construction cost.When I delayed to pay one instalment by two months he charged 18% interest rate as per agreement. My question: I have paid 50% of the total cost , but the building is constructed by 20 to 30% only.The bulder has to provide me swimming pool /gym /shopping mall / club house etc all are not constructed. Is the builder has the right to ask me interest on delay in payment as I have already paid higher proportionate of money compared to building construction cost ?