I own a 19th century house in upstate New York. It does not have a real basement, just a partial, rough-built cellar. Recently we noticed that the walls of the cellar were seeping water during heavy rain. We had a contractor look at it, and he said it could be repaired by digging down on the exterior and sealing it from there. After digging up our yard, he said that the foundation is made of loose stone (no mortar) and that we need to replace the entire thing. Is this really necessary? How can I be sure?