Hmm. OK, this won't be precisely foundation height, but it's close.
First, make sure your floor area is clear and delete any floor or the roof above where you want the stairs. Otherwise, you'll get a ceiling is too low message.
Now:
- Ctrl-shift-C, type placefriezes on, enter
- Use the foundation tool to place a frieze where you want your landing.
- Page up to the new 'second' floor, which is the top of your frieze.
- Select and place the stairs to the lower level. (They will automatically form downwards, not up as is normal.)
- Page up to the new 'third' floor, which is actually your true second floor.
- Put a floor tile where you want your stairs to end. This may take some experimentation to get the correct distance.
- Place the stair to the 'second' floor, AKA the top of your landing.
- Ctrl-shift-C, type placefriezes off, enter.
From here, you can proceed as normal. Keep in mind that the stairs will not form the standard walls beneath them. I have not tested whether they will still function properly if left as a 1X1, though they will place. You may need to do a 2X1.
EDIT: I have also not tested this when placed up against a wall. You may have to place a wall on top of the frieze and flatten it out using constrainfloorelevation false and the levelling tool. Just remember to set constrainfloorelevation back to true before you do anything else, or you can throw your building greatly out of whack.
