Game guide:Building a basement using foundations

Small houses can be frustrating to play with, especially with large families of Sims. On top of this, making additions to the house may not be practical or desired, due to lot size, desired architecture or aesthetics. Fortunately, houses built on foundations are able to be remodeled to include a basement. However, prior to the "Basement Tool" (included in versions 1.6.6 and later of The Sims 3), the process to construct basements is not explained in-game. As a result, some players may not understand how they are constructed. Fortunately, building basements is quite simple, and can be done to any pre-existing house on a foundation, or to any new construction.

Step-by-step instructions
For the purposes of this tutorial, an empty lot has been chosen to demonstrate the construction of basements. If you are remodeling an already-existing house, move to Step 2.
 * Step 1 - Select your site

Ideally, Basements should be built on flat lots, though there are exceptions to this. For beginner builders, level out the lot before building the foundation.

To start, lay down the foundation for the building, as you would if you were building a house without a basement. You can begin to build the above-ground portions of the building right now, or you can choose to wait until the basement is complete.
 * Step 2 - Lay down foundation

Use the foundation tool and hold the Ctrl key to remove the foundation, excluding a one-tile-wide foundation around the perimeter of the future basement. The basement does not need to be as large as the house above it, so you can leave some parts of the first floor foundation alone if you do not want a basement built below it.
 * Step 3 - Hollow out the foundation

Note that if you're adding a basement to a house, you may want to copy the floor tiles with the Eyedropper Tool, since removing the foundation will also remove the floors on top of it.

Find a wallpaper or other wall covering to place on one or more sections of the foundation inside the hollowed-out area. The covering chosen does not matter, except that it should have a pattern, a baseboard or some other feature that will allow you to determine the height of the wall.
 * Step 4 - Paint the foundation

Using the lower land tool, start digging a hole near the foundation that you've painted/covered, exposing the entire length of the painted wall. Use the patterns or baseboard on the paint you've chosen to help you find the depth of the hole you're digging.
 * Step 5 - Start digging

Once you are able to get a tile of land inside the basement to the correct level, use the level land tool to lower the rest of the land inside the hollowed-out foundation to the same level. Afterwards, look around the outside of the foundation perimeter to make sure you didn't create any holes.
 * Step 6 - Level out the basement

Once you've reached this step, the basement is already usable. However, because foundation walls are not treated as normal walls by the game, it's useful to remove the foundation around the perimeter of the basement, and to place walls down.
 * Step 7 - Remove the foundations around the basement

The walls placed in this step are normal walls, and can have objects placed on them. If the walls are not connecting to the foundations correctly, the basement depth may not be right.
 * Step 8 - Place walls down around the basement perimeter

Once the basement walls are built, rebuild the foundation around the base of the building.
 * Step 9 - Rebuild the perimeter foundation

Finding a good location for the stairs may be difficult. You might be able to find a good spot for the stairs by placing some of the walls on the main level.
 * Step 10 - Place stairs

Complete the basement, just as you would other rooms in the building. Your basement should now be finished!
 * Step 11 - Finish the basement