You may be correct in your statement as to the legal rights of the case.

I take it for granted that you are.

At any rate, I know nothing about that; and I rest no argument upon it at all.

But it is clear as daylight to me that morally you are bound to give her the money.
Suppose you had had permission from her to make those changes in the house, while you were still her tenant, and had found the money, then you would have handed it to her unhesitatingly.

Why?
Because you would have said, 'This woman's husband built this house.