Put down staircases on a new lot to lower the terrain. Go to the bottom of the first one, and miss out one tile, then put down another staircase.
Repeat this again to have three staircases. Delete the staircases. Use the “Level Terrain” tool to level the terrain from the lowest tile.
Set it as big or small as desired. Using the “Straight Line Wall” tool, build walls around this hole so there are walls pressed up against hills. Build the room above so it covers the entire basement.
Cover around the house with the “Foundation” tool; all the slopes should be covered up.
Place one tile in the top floor, then create a staircase leading down to the basement.
Make two or more foundation blocks at the front of the house, and place a staircase leading up them.
Here’s a video on creating a simple basement in the Sims 3
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