Firstly build a lot as shown with two levels exactly the same
Turn on the cheat boolprop constrainfloorelevation false
Page down and add 1 square of foundation as shown (depending on which way you want your staircase, reverse for opposite effect)
Using the small raise terrain tool, raise the ground (corner of next landing, as shown) by 4 clicks. Please count correctly, it must be 4 clicks for the staircase to place properly.
Using the foundation tool, drag from the raised corner as shown, placing 1 square of foundation.
You should now see below - notice how the first landing has warped?
To fix this warp, place 1 square of floortile as shown (on the grid next to the foundation, not the ground underneath)
The floortile forces the warp to flattern
Now go around the back of the lot. We need to flatten the walls/roof area.
Using the level terrain tool, drag it from the corner (the area over the normal height foundation) diagonally over the rest of the lot, as shown
Now turn the cheat off boolprop constrainfloorelevation true
Using the level terrain tool, level the ground where we raised by 4 clicks
Now we can add stairs
For the final piece of stairs, firstly we need to add a floortile to the second level in order to be able to place the stairs
So level up to the "top floor" and place a floortile as shown - 4 spaces/squares between the walls and floortile - place floortile on the fifth square (directly inline with the raised foundation landing)
Now place the final piece of stairs
And you're done. You can now add the second level of walls to your lot.
Please be aware that the space above the stairs should be open, i.e. don't place floortiles there.
Any questions or help needed, ask below xxx