Illegal building

Check Commencement Certificate of the bulding, How much built up area, floors, and land area mention therein, check building plan approved by concern authority/s, compare areas of commencement and approved plan also check Completion Certificate whether areas of commencement,  Approved Plan and Completion  and actual building areas are tally, verify FSI , TDR  consumed for cstruction, Structulral Design need to be verify.   Also verify whether Land is situated as per demarcated Lay-Out (Approved), Zone Certificate, Also check out State rules/Corporation rules/Bye-Laws according to that building is completed or you can go with stability certificate or Architect Certificate to verify about building. also all compliances are completed or not need to be verify, like fire fighting licenses, lift etc.