A polishing apparatus for polishing semiconductor wafers comprises a main polishing structure, which includes a plurality of polishing tables, a plurality of polishing heads and a plurality of load-and-unload stations, and an add-on polishing structure, which includes an additional polishing table and an additional polishing head. The add-on polishing structure can be attached to the main polishing structure to form a larger polishing structure with the additional polishing table and the additional polishing head.