Create a base and type the code "Constrainfloorelevation off" ("on" to disable it)
Add a floor
Put a floor above the added floor
Add a staircase to the floor itself 3 times (as in the photo below) or lower the floor level by clicking 9 times.
Click from the lowest area and stretch towards the ends of the base
There you go, you've just created a foundation over a wall, even faster and easier!
Here is another example of what you can now do
