Difference between sanction plan and site plan of flat

Dear Sir/Madam, I am on the way to purchase a 1 bhk resale flat which is 3 years old. The owner has an OCCUPANCY CERTIFICATE from the local body. But the sanction plan and site plan is different. Sanction plan has two bhk layout, but site plan is 1 bhk , which is in reality a 1 bhk flat. The seller says that occupancy certificate is granted on the basis total covered area, column structure, floor space ratio,etc,. Seller comments that a site plan can have less no. of room as per requirement, but total covered area should be as per the sanction plan. Please guide. Regards, Somnath