The constrainFloorElevation true/false cheat is an incredibly versatile tool for building purposes, especially in The Sims 2 and The Sims 3.
The Sims 4 already has split-level foundations and L-shaped/U-shaped staircases, but no ability to make walk-out basements. I have actually seen a tutorial for a walk-out basement in The Sims 4, but the main problem is, it is just a sunken room, thus the ground must be covered up with shrubs and flora.
It would be much nicer to have a walk-out basement or an editable foundation that allows a walk-out basement. Maybe do a basement overhaul or something.