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.
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 can be as deep as four stories, or two stories deep in The Sims 4.
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.
Game Guide:Building a basement using foundations, for constructing basements using the foundation tool in The Sims 2 and The Sims 3.