Regrettably this is a health and safety issue enforced by ambulance chasing lawyers. More years ago than I care to think, I decided that I couldn't stand the enclosed staircase I had in my own house. I thus opened it up fitting the bannister rail on the wall. A few years ago I developed an incurable disease which gave me much walking difficulty and my LA social services department tut-tutted about my lovely open staircase and sent a man around to enclose it again. It took me a mere two weeks of increasing anger as to the appearance of the staircase before my electric screwdriver came out and on hand and knees the offending bits of timber were removed!
My tenants of course all have to put up with enclosed staircases.
Any information given in this post is based on my personal experience as a landlord, what I have learned from this and other boards and elsewhere. It is not to be relied on. Definitive advice is only available from a Solicitor or other appropriately qualified person.