Have you had a look at this thread here. You might find an answer there.
Thx, but that doesn't slve the problem. In the order version, when I delete the fundation, I can build the stair. But now, no matter how many tile of fundation I delete, the fundation is still "there" because it says can't be placed inside fundation.
I also discover I new problem... If I lower the ground too much and build a basement based on the lower level, can't be placed inside fundation message will come up, too.
Sims 3 is really getting more and more frustrating.
NVM, found the solution. need to build walls on the ground floor in order to make stair works.