Architect's floor plan

My floor plan

You can use beams and/or posts to open up the stair. The advantage of walls is that they are cheaper and allow greater separation and storage. Don't forget the stair is going up through a floor/ceiling opening so not all of it can be exposed.

Ask your architect. There are other factors involved and the architect would know how it was designed and potentially be there to investigate what and how it was actually built.

Thank you all for the feedback. I was told that walls are cheaper than rails but am not sure how much cheaper. It's great that walls are not the only option.