SimSweet

Step 1:

Create one square of outside foundation. 

Step 2:

Put paint on the foundation. Make sure there is a bottom trim. It will become really important in the next step. 

Step 3:

Using the lower foundation tool, lower the ground untill you see that bottom trim. don't go any further otherwise it won't work. 

Step 4: 

Delete the foundation. Now using the room tool, create the shape of your house starting in that part you just lowered. 

Step 5:

The final step is to put a boarder of outside foundation around the basement walls. Make sure you place stairs so your sims can get to their basement!