Adding an Interior Door on a Stair Landing
There shall be a floor or landing on each side of a door. Such floor or landing shall be at the same elevation on each side of the door. Landings shall be level except for exterior landings, which are permitted to have a slope not to exceed 1 unit vertical in 50 units’ horizontal (2-percent slope). - Florida Building Code 1008.1.4 2007 - See more at: http://inspectapedia.com/Stairs/Stair_Landings.php#DoorSwing


There's all sorts of code requirements that will not allow a door to be put in there. If you must have a door and not a curtain and you don't care about code, then I would put the door like this:



You'll have to install a door jamb, I would be surprised if you found a door from a big box store that perfectly fit so you might have to custom build it all yourself. It would be a big project!