My parents' old house had a wonderful oak front staircase that my six siblings and I used to race up and down, taking them two or three at a time, with only a casual hold on the railing. I don't know how we managed to never slip or fall. Now, the timber backstairs were steep, and had narrow treads and were poorly lit. We NEVER ran up or down them, but held tightly to the railing and hoped to heck we managed to stay upright. As an adult, my previous home had carpeted stairs, which I slipped on (wearing socks) and fell down twice, though thankfully I was closer to the bottom than the top. My current home has timber stairs with three narrow stainless steel runners stood on edge and inset horizontally across the front of each tread (this is supposed to help reduce risk of slips). I doubt I will get over the anxiety of taking another tumble, so I never go down them wearing socks, and if barefoot, I hold onto the railing for dear life and take it nice and slow. So far so good.

Agree with all above. First think what you need the extra space for - you have lots of options. Also remember it is a walk through, so a 'public' space. If used as a study would be best if not too messy or private.

Thank you for your Comments. It was the under-coat primer we used. And the client opted to paint it Black eventually in his spare time.

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