I was thinking of making stairs too... but I scrapped the idea because of the space they take up - and I fail at making modular staircases inside.
I don't know whether you have Late Night or not, but if you do there is a way to get elevators working with foundation instead of the stairs.
To build an elevator through the foundations of the house choose the place to put the elevator and delete a 4x4 area of the foundation.
Using the "moveobject on" command place one of the 'inbetween'-pieces (no idea what they are called in the english version) in the middle.
If you cannot get the location exactly right, maybe because of it snapping to slots anyway use the "disableSnappingToSlotsOnAlt off" command,
hold down the alt-key and position the object.
Now go up to the ground floor and down to the basement - place you elevators:
I know these aren't stairs, but maybe you will opt for this comfortable method
I had the terrain raise up suddenly and had to use the "constrainfloorelevation off" or false command and level tool to smooth it out again. (no screenshot of that though)
EDIT: The pics don't show? And this was shown to me as a new thread for some reason... and I didn't check the date - sorry about this.