I dont think you can...
I dont know if this works, but you could try going on landsculpting, lowering the ground level, making the ground level even and then try and put walls on it.
I just made that up right now, I dont know if it works... a basement is a good idea, I never thought of that. You could always build a garage if it doesnt let you build a basement. :)
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Yes the sims 3 is out on Ps3
press start and go to buy and then it will say build hope this helped.
Can it snow on Sims 3 ps3
How can I have boy girl twins on the sims 3 pets ps3
No but they are coming out w/ sims 3 and I'm almost positive it's for the ps3