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.

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.

Official
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. 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.