Beware of buyers. the buyers shall verify all necessary credentials in the document. Without site building cannot be constructed. So there must be undivided and unspecified of some extent of area of site. How much of total builtup area of your flat? You subtract your flat builtup area with 9, then the answer is your extent of site(unspecified and undivided out of total extent of the entire site). But it should be mentioned in the document. so get a rectification deed from the seller.