1) terms of contract are sacrosanct
2) if builder has agreed to give additional 25 per cent carpet area it is binding upon the builder
3)builder has to give you additional area at 20k as agreed upon
4)if builder is refusing to honour his agreement you can file complaint before consumer forum against builder and seek court orders to provide 25 per cent additional area as agreed upon
5) seek litigation costs and compensation for mental torture undergone by you