The Stair Tool cannot create stairs that go under itself. One option is to create the stair in two parts.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
If anyone has good links to the User Guide, Reference Guide, other tutorials or videos for the stair tool (AC23) that would be appreciated. I jumped in on the stair tool today, it seems like every new version of AC that I use they completely re-vamp the stair tool to make it harder to use. I can't figure it out at all and I've used AC since version 6. But in defense of Graphisoft, I've also aged as they've progressed....
I understand the basics, which are consistent, but as I tweak the details I can't create a final stair that is of a very basic design.
I'd say stick with figuring out the stair tool. I considered the other day doing just that, building it up all out of slabs, walls, roofs, etc. That's pretty messy and defeats the whole point of BIM. So I persisted, it's super tedious and not at all intuitive but I'm starting to figure it out. Not to understand it, mind you, but to at least figure it out.