Builder asking for more money

Hi,

My builder in Gurgaon has offered possesion but is asking for more money saying that the super area has increased. He has mentioned that :-

As per terms of the Floor Buyers Agreements, the area of the residential unit was tentative and was subject to change. Now that the constuction has completed and the final area has been verified from the site, we would like to inform you that the super-area has increased

The increased amount deducted is substancial.

Is the builder legal in asking for more amount ? Can I file a case in consumer court against him?

Thanks and regards.