I built a house today which has a basement and sits on a foundation. The basement was built first, then the stairs up from it, then the foundation, stairs connecting the basement stairs to the foundation, then the rest of the house. I then placed another set of stairs over the descending pair which lead to the first floor (which doesn't actually exist yet, they only lead into the roof) and done was done. However, I just tried to go downstairs to the basement and my sim just can't. The action is canceled, the footsteps and crossed out sign thought bubble appears etc. But I don't understand what the issue is. Here's some photos of the stairs:

The ground floor where my sims refuses to go down

The foundation level, that grass square can't be deleted so I'm wondering if thats the issue

The basement level, I tried deleting the walls around the stairs and originally the stairs where enclosed in a room where the wood tiles show

Any advice?

You should post this in the create/sims3/building.

There has been a few patches along the way. Last time I checked I couldn't build staircases above each other like that.

I just wanted to clarify some things before commenting, because there seems to be some confusion about which level is which.

The "ground floor" (screenshot #1) is the Foundation level. It's not the ground floor. Remember, the foundation is the floor of the first house.

The part of the house that you call the "foundation" level (screenshot #2) isn't the foundation. It's the Ground level, what the foundation sits on. That's why you can't delete the grass tile. It's the ground itself.

I'm a little confused by what I'm seeing in the first screenshot. When you build a staircase in a house that has a foundation, there's supposed to be a little landing at the Ground level to give the sim something to stand and walk on before going down the basement staircase. I don't see anything like that in those screenshots. If you didn't create a landing at the Ground level, then yes, your sim won't be able to go down the basement steps at all because there is nothing for her to stand on.