I am digging footings for a 2 storey extension and I have taken the external wall footings down to the level of the bottom of the original strip foundations, which is 1m below finished floor level. I have a 100mm internal block wall which will carry the first floor joists. Does this footing need to be as deep as the external walls, which are 300mm cavity to 2 storeys. The building inspector is coming monday and I am hand digging the remainder of the footings this weekend. I know that I have to go to good ground and this appears to be about 700mm below ffl. External walls will have 500mm trench fill.