Developer asking more money for increase in super built up area

Dear Sir/Madam, I booked a flat in one of the projects in Nagpur with Empyrean sometime in 2012. The unit area which I agreed was 1050 Sq ft in agreement, I paid all the amount (21 Lakhs + Taxes) all on time, and at the time of possession they ask for 1 Lakhs more saying there was increase in super built up area for my unit by 50 Sq Ft. There are couple of things to mention here 1)Builder or Developer didn't put me in loop about increase in area, they mentioned at the time of possession. No question about taking permission but not inform me earlier 2)The carpet size is same, the actual increase is in super built up. 3)The area which they increase is Balcony, and i can understand they did this to make extra money Here is what I signed with them as ATS clause which reads as under. 3 (a) It is understood and agreed by and between the Parties that the lay-out plans of the said Project as seen and accepted by the Allottee(s) / Purchaser(s) are tentative and subject to alterations and modifications by the Vendor Company, the Architect and / or any competent/sanctioning authority before or during the course of development and the Allottee(s) / Purchaser(s) agrees that the Vendor Company shall be entitled to effect suitable and necessary alterations/modifications in the layout plans without any objection/claim from the Allottee(s) / Purchaser(s) and it shall not be necessary on part of the Vendor Company to seek consent of the Allottee(s) / Purchaser(s) for the said purpose. (b) The dimensions / sizes / carpet area / unit area of the Said Residential Unit are subject to variations within reasonable limits and may vary up to 10% of the size as stated above. The points which are raising question in my mind are. 1. How can they ask for increased super area? 2. What option I have here, can i go against them in consumer forum or send them legal notice or pay them the amount they are looking for?