Builder demanding exorbitant interest to give possession

I was allotted a flat in a township close to Delhi and now when the flat is ready the builder is demanding a very heavy amount of interest alleging that I had opted for a down payment plan. This is ridiculous as I was not provided any discount or any other concession and also did not choose the down payment plan. The builder has also increased the super area of the flat substantially and is also demanding the additional amount for the increased area. Please advise what steps should be taken to curb the Unfair Practice adopted by the builder and obtain the possession of the flat allotted to me. Should I lodge my complaint with the consumer forum?