I'll try to quickly explain how I build a SIMPLE no frills basement in all my houses. Just takes a sec.
I always build my house first. I get my roof just right, before I build anything else. So here we have a house (this is a recreation of a house I saw on Sims2Community forum that I liked).
Now go down to your foundation level.
Now, using control and your foundation tool, delete foundation for your basement, always leaving at least one whole foundation around the basement area.
Then I toss on some flooring. Be careful here to make sure you are flooring your first floor and not some floating floor tiles that sometimes appear. They disappear once you get the first floor filled back in.
Now, go down to your basement level. Choose a wallpaper with just a floorboard. I like this green one with white floorboard in the Painted walls section.
Cover a wall, or all the walls. Whichever.
Now, get your terrain lowering tool and start carefully clicking near your painted foundation wall. A friend mentioned that you must be in walls up or cutaway mode, not walls down, or else your wall will lower too (thanks JenM on LL!) . I also click some of the surrounding terrain down, click by click, to more easily see the basement wall I'm uncovering.
Now, when you just barely see the white floorboard for the first time, STOP.
Get your terrain leveling tool.
Put your pointer in the corner right by the uncovered white floorboard and drag out the floor level a bit.
Now level it all through out the basement, making sure to only level within the foundation area you deleted.
Cover your new floor level with tile/flooring.
Go up one level, delete 5 spaces for stairs making sure that what you delete is over open space, and not foundation (you won't be able to build your basement steps right on the edge of your house or anywhere you've left foundation intact).
Grab your modular stair tool and click your stairs down to your basement. Or use the full stairs thing, placing them in the basement up. Whichever. I ALWAYS use modular stairs because more sims can use them at one time.
You will not be able to hang objects on your uncovered foundation walls unless you use move_objects on.
However, all internal walls you build yourself will function like normal walls and you can hang things on them normally.
Finish off your stairs with fencing, walls, or however you normally do or want to do. You may also repaint the basement or unpaint the basement foundation walls (to leave the plain foundation brick).
There are a lot of tutorials on the Sims 2 exchange for more advanced basements (windows, walkout, driveway, etc). Go to the stories, enter keyword basement, change the category to How To/Tutorial, and hit go.
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