Instructions
Open "Sims 3" and navigate to "Build Mode" then "Foundation." Drag your mouse across the ground to place the foundation.
Press "Ctrl," "Shift" and "C" simultaneously, then enter the code "ConstrainFloorElevation False." Navigate back to "Build Mode" and select the "Wall" tool. Build the building walls next to the wall foundation. The walls will automatically connect, however, one wall will be taller than the next.
Click the "Level" tool and drag it across both the building and the wall foundation. This will make the building walls short and level with the foundation.
Select "Walls" then place new walls around your building. The walls will now be level with everything, however, you will be unable to place a door correctly. Fix this by deleting the bottom section of the wall with the sledge hammer, then adding a new wall with the "Walls" tool. This will cause part of the building to become higher than the rest.
Fix the building height by selecting the "Level" tool again and dragging it across the part of the building that needs adjusting. The walls will now become even.
Press "Ctrl," "Shift" and "C" simultaneously then enter the code "ConstrainFloorElevation True." Navigate back to "Build Mode," then select a stair case to attach to the building foundation.
Navigate back to "Build Mode" and select "Doors." Place a door in front of the stairs so your Sim can enter. You will now have a functioning wall foundation attached to a building.