I started building a new house yesterday, off a floorplan I got online. I was a little over-excited to start building, and I forgot to put up a foundation before I started putting up the walls. Whoops. I do this almost every time I start a new house, and usually I just tear down the walls, put a foundation, and then rebuild the walls, but I'm tired of doing that. Is there a way to put in a foundation without taking down the walls? Or am I stuck with having to take down all my walls?

You can use the powerful cfe or constrainfloorelevation false/true cheat and rearrange the height of your walls. It's a bit buggy though, perhaps leave the ceilings unpainted. If you put down a foundation beside your house, type the cheat and then just drag the flatten terrain tool straight across your house from the foundation you put down. Turn the cheat off using true instead of false immediately, cfe is not something you want on while building in general.

If the other walls has been rearrange as well you can just put up a regular wall and drag the terrain tool across again.

It doesn't give you a foundation, but after using cfe like that you can replace the walls with foundations if you like.

Thank you so much! I will try that.

However, this still means that everything you've placed on the former first level (which now becomes the foundation) will need to be placed on the second floor (which now becomes your first floor), and everything on the second floor will need to be on the third.

You'd be better off doing an earth foundation (basically raising the terrain the house sits on and placing a staircase in the front. With some smoothing, it looks just as good!

Yes, it depends on how far you've got on your building.

Still using cfe you can also raise the foundation to the same level as the new basement floor and tuck it around it. Then you can raise the terrain. I think you have to use cfe to raise the foundation, but it may be possible to just drag it from the walls. I think it may depend on your patch level.

Just "upgrade" to The Sims 4 in a couple of weeks...

This is what I thought when EA developed TS4. EA builders always forgot to build foundation prior to walls, hence this "feature".

It's a part of life really. If you need a basement, plan for it. Doing it after can be a lot of hassle, will cost you tons of money and won't look as good, just as Sims3. TS4 isn't even comparable at the moment without cast, and talking about that game here is just annoying.