Basement

$$ Basements are stories of a building built partially or completely underground, or below the ground floor of a building. Basements can be built underneath some buildings in The Sims 2 by using the foundation tool, or by using the dedicated basement tool in The Sims 3 and The Sims 4.

Traditional way
Basements in The Sims 2 and unpatched versions of The Sims 3 can be created using foundations by placing an area of foundation that is hollow in the center, then lowering the terrain inside the hollowed-out foundation so that the area in the basement is the equal height of a typical floor. This makes the basement a fully usable space accessible by Sims, so long as the Sims have a means to reach the basement.

Basements are not possible at all in The Sims, as that game lacks both foundations and a dedicated basement tool. The basement tool was not included in The Sims 3 for console, but a basement can still be constructed using foundations in the same manner as in The Sims 2.

Basement tool
Following the release of the first expansions, update 1.6.6 for The Sims 3, and patch 15 for The Sims 4 introduced the Basement Tool into build mode, allowing the construction of basements on houses without foundations. This tool was used to construct the tombs included in The Sims 3: World Adventures. Unlike the ones built with terrain tools, these are recognized as additional story levels below ground, therefor cannot be seen from above viewing level. Also, the vacant area outside these basement rooms is surrounded with the empty black void. The basement tool can be used under foundations, but two sets of stairs will be required to reach the basement area, as a foundation cannot connect directly to the basement floor. Basements in The Sims 3 and The Sims 4 can be as deep as four stories.

To build a basement in The Sims 3, the basement tool can be used to zone in an area for a basement to be constructed. Both straight and diagonal boxes can be zoned in using the tool. A basement that is constructed using the basement tool can be accessed with the camera by using the floor controls to change focus of the floor to the floor below the ground level. Additional, deeper basement floors can be built underneath the first floor of the basement using the basement tool.

To build a basement in The Sims 4, a wall must be placed in the lot. After the wall is placed, the basement can be accessed by changing floors to below the ground floor using the camera, in the same manner as changing to floors above the ground level. Lower floors are unlocked as walls are placed at the lowest basement floor in the lot. No pre-zoning of the basement is required in The Sims 4.