I've searched and found NO answer for this problem.
You can't place through a foundation, fine, and I see there's a "continuous staircase" solution, but the problem here is... there is NO foundation. No floor tiles, no objects, no walls in the way of the landing, nothing, there's plenty of space, etc. I've tried with it completely blank, not building or placing a thing above the basement, and it still says there's a foundation in the way!

See for yourself. Am I missing something? (It's off to the side of the house to show there's NOTHING around it. At all.)

http://i52.tinypic.com/2gtq4ao.png
http://i54.tinypic.com/20pd7hw.png

I just tried it on a different lot (different save file) and same thing happens, every time.

Build a box very close around where you want your staircase to go. Then remove the box and build your staircase.

This is just qualified guesswork from me. It sounds like the wall above where you want the staircase is too low. Doing this you lift the hight definition of that area to a regular wall hight.

To be on the safe side you can build a big box around your staircase, then using cfe and any tile, drag a floor above your new walls to build a roof. This should lift the hole area.

^This changes nothing. :/

if you don't use the hand rails with your steps will it work? You don't have CFE on right?

I would probably just try placing inside basement, inside house, with or without cfe, with or without moveobjects on, turn the computer on and off. If I still can't find anywhere to put it, I just have to assume my downloads, or EA's game have a bug.

Have you ever tried placing your basement before anything else, and then put down the stairs before any walls?

Becquerel, in your first picture it seems as if you are down in basement or something. Could be wrong tho, just the black surrounding your room tells me your picture is of a room in a basement type area.

Also, have you tried removing all roofs?

cheers

PS: Maybee there could be some mod interfering with things? Have you any mods etc or maybee something corrupt you have downloaded into the sims 3?

I'm having the same problem so I immediately tried using the "ConstrainFloorElevation True/False" and the "moveObjects on/off" cheats (and combinations of those, like one on and the other one off) since I've been using a lot of cheats to build somewhat weird stuff on that lot, the thing is... it didn't work. Either it still wouldn't let me place the stair (to allow me to connect the basement to the upper floor) or it would place the stairs leading from the basement to a non-existent even lower floor (wth?) and thus placing useless stairs that lead nowhere.

IF however I try placing those same stairs, in the exact same area, but connecting the ground floor to the 2nd floor, the stairs are placed just fine though.

edit: I can place the stairs just fine when I first start building this house. The issues comes when after building a few more stuff on the lot I need to remove the stairs and then rebuild them. I know there's nothing wrong with that area 'cause I can place the stairs on that spot the st time I do it. That's also how I know (or think) I have no mods causing this

To be able to place stairs into a basement like in your first picture, the height of the room above it (ie ground level) needs to be at least a normal room rather than foundation. I would recommend going up a level and building a room around the area and ensuring it is high enough before trying again.