Are you building a nifty house on a foundation and have later decided it needs a basement?* Most of the time builders think going back and adding a basement will require them to rebuild the house from scratch. Not with this tutorial.
Did you plan to put a basement in but didn't know where, drew out your lot, deleted the foundation but the game still won't let you place the stairs down into the basement? Again, a lot of time, most builders restart the lot because the game engine thinks there is still a foundation, even when it's been deleted.
This tutorial is a simple lesson on how to get stairs down to the basement level on a lot that already has a foundation. It involves NO cheats so if you're not comfortable with constrainfloorelevation or just don't want to deal with it, this is perfect for you!
A. You have your awesome floorplan and realize that you forgot to build the basement.
B. Decide where exactly you'd like your stairs. It's best to mark this in some way. Once you've marked it, delete the walls around it, leaving a one tile space around it.
C. Delete the foundation where your stairs will go, following the same one tile space.
D. Now, using the wall-room tool, draw a box in the hole on GROUND level. Because you have space between the wall and the foundation, you do not need to use cheats.
E. You should now be able to place your basement stairs without issue inside the newly drawn, ground level room.
F. Delete the walls.
G. Using the foundation tool, draw the foundation back in. The game will give you a "can't intersect modular stairs" error. Don't worry, it will place the foundation everywhere it can, without affecting the stairs themselves.
H. Fill in the walls you deleted in B and make sure to make stairs from the foundation to ground level. You're done!**
See, wasn't that easy?
*This will also work with multistory homes. It's easier if they're not already decorated but all you need to do is move the furniture so you can delete the foundation.
**The game will let you place stairs as shown below to look like one full staircase. DO NOT DO THIS! If you do, your sims will be unable to access the basement. Stupid, yes. Unfortunately, I don't know a way around this one unless you teleport them down there and back up.