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 and The Sims 3 by using the foundation tool, or by using the dedicated basement tool in patched versions of The Sims 3 and The Sims 4.

Foundations
Basements in The Sims 2 and 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. It is still possible to build basements with foundations in versions of The Sims 3 that have the basement tool.

Basement tool
Patch 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 buildings without foundations. The tool was previously used to construct the tombs included in The Sims 3: World Adventures. Basements created using this tool are considered an additional story level under the ground, and when viewed the surrounding area is black, rather than showing the terrain. The basement stairs cannot connect directly to a foundation in The Sims 3, but they can in The Sims 4. Basements in The Sims 3 and The Sims 4 can be as deep as four stories.

The Sims 3
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.

The Sims 4
To build a basement in The Sims 4, either the basement tool or custom basement tool can be used to draw rooms, similarly to how above-ground rooms are created. However, if the player would like to build a basement a wall at a time, they will have to change the focus of the floor to an underground floor, which requires that there be at least one wall on the floor above (though it can be deleted after being placed). No pre-zoning of the basement is required in The Sims 4.

If a basement is built under a raised foundation using the basement tool or custom basement tool, it will cause surrounding terrain to "jump up" to the top of its walls, as the basement is slightly raised out of the ground. To avoid this, players should instead build basements while focused on an underground floor (using the wall tool or the room tool), so that the foundation will not influence its height.