Dismantling old houses is always going to be problematic. Mildly if you are lucky. In our case at Frederick Place we have uncovered a few little treasures including a well....under the bath. We have had to excavate atleast half a metre below ground level to check the footings on all exterior and party walls.
In doing so, in the existing bathroom we uncovered a little hole, which turned out to be a big hole.
So, with advice taken from the structural engineer we back filled and cemented, problem solved.