How much does it cost to raise a child? Same type of question.

[Bruno1949]

Being a registered historic home all exterior ‘improvements’ will probably come under some sort of council vote or at least a lot of paperwork. This will complicate, delay, and increase the cost of any repairs visible from the outside. That could include everything from the type of fence or the kind of sidewalk or trees and bushes you can put in, to the color of the trim.

As for the falling foundation it could cost anywhere from hiring a brick mason to reinstall the original bricks, fairly cheap, to replacing the entire foundation after you jack up the huge house to get to it. That could cost $100,000. There is no way to tell from your description. No matter what, restoring a home like that will cost many many thousands of dollars.

It won’t be quick. It won’t be cheap. It won’t be easy. Figure on years, not months.

If the foundation is failing then the rest of the systems are probably failing too. New wiring. New plumbing. New furnace or boiler. New chimney and roof. Rotted or damaged studs and joists and window and door frames. The list can go on and on. Without a building inspection and full report you have no idea what you are looking at in the way of time and cost. And neither do we.