sell agreement and sell deed

I have done agreement of new build flat in apartment, builder says that he made agreement based on carpet area , carpet area is 413 sqf (hall+kitchen+bedroom+bathroom+toilet) and two balconies of 82 sqft in buitup area.he mentioned overall 495 sqft. Built up area agreement. He did not mentioned lobby area in agreement but he says that it is in property. I want to ask that how builder can include this lobby in my property if it is not mentioned in agreement. He take some money in cash from me as black by mentioning low value of my flat in agreement for that he omitted lobby area he do that for minimizing government tax but in which I only save 40,000, Finally I want ask that is lobby legally in my property ? And if not then what is the way ,i have made payment for that also and only few lakh of white money is remaining I have not done sell deed