The only way to be sure is to dig down beside the wall and see if there is a footer there. Many homes that old were built with the foundation wall on top of a make shift footer of stone or brick and mortar slightly wider than the wall. I have seen them where the wall has no footer at all. If you encounter an area where the wall juts out, or where you no longer see the foundation wall, that is the deepest you should go. There is no way of knowing how much headroom you will gain untill you find the bottom of the wall or the top of the footing.

Glenn

So lets say that I don't find footers under my foundation walls. How far down could I safely dig under my existing basement floor?

If I *do* find footers, is my previous research correct in that I can dig down to the top of the footers without worrying about collapsing the basement?

Thanks for your help!

Yes you can dig down to the top of an existing footer. If no footer is found under your walls do not dig lower than 8 inches above the bottom of the wall. You could go down to the bottom of the wall but temporary shoring would be required to brace the walls until a concrete floor slab (consisting of a 4" stone base and 4" of concrete) is poured to stabalize them.

Glenn

If you try digging to the top of an existing footer, and no footer is there, you'll sooner or later get to the bottom of the wall, where you were just told not to go.
Try running a metal rod down near the wall at various places to see if you can detect a footer.