The required depth of footing for a 2-storey extension is dependent on the ground conditions. In reasonable ground conditions, it is not unusual to excavate 1000mm deep x 600mm wide. Your local contractor will have experience of local ground conditions and will dig to a depth that he feels is suitable. The building control officer will inspect it and approve it if is satisfied. If not, further excavation may be requested. Where there is made-up ground, a raft foundation might be recommended.