@mon_graham, You mention that you are building and that currently specified in the plan is spotted gum as oposed to the oak floor boards that you would prefer, well if it's a current build, can't you simply put in a change to the builder??? If you can't change the plan then try to incorporate some spotted gum in the room by adding some hardwood furniture, and then the stairs would tie in with that room. Cheers, Barbara

Japan black the stairs and paint all white except for the gal angle. Actually I cheated with the Japan black. I made a black stain from water based black interior paint. 50/50 water and paint. Just like the karate kid - I wiped on and wiped off :) I used one rag to wipe the stain on and another to immediately wipe it off. I then I painted on 5 coats of modified tung oil. They look sensational. Pictures dont do my stairs any justice at all. Part way through staining Stained: Painted white: With hand rails: Ignore the landings. They are about to stained and tung oiled very shortly

You could add a chaise lounge or a comfy chair and table for a reading nook or an office area if you need one, a computer area but whatever you do don't over clutter it keep it sleek and sexy and or a comfy chair as suggested with a little side table for reading, I also like the idea of a place to put coats, shoes etc if you must enclose it. Maybe even a small wine bar? with a couple of stools. I decided to google is and will give you a link to many pics to give you more choices. https://www.google.com.au/search?q=pics+of+what+to+do+with+space+behind+floating+stairs&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjFy-n9p4vZAhUEF5QKHSvhAOMQ7AkISg&biw=1280&bih=893

I agree with Macyjean about the builder not wanting to do it, rather than it can't be done. However "save it until later" solutions often never get done and you do need a balustrade or something to make the stair compliant. I would look at removing the existing wall and rebuilding it abutting the stair. Put the extra space into the powder room which will feel much more spacious. Space is the ultimate luxury. Best of luck Dr Retro of Dr Retro House Calls and Dr Retro Virtual Visits via Zoom