It's an illegal merge, it's highly unlikely the made sure to do the work "up to code". That being said, if you look at the railing it looks to be made out of a

It's an illegal merge, it's highly unlikely the made sure to do the work "up to code". That being said, if you look at the railing it looks to be made out of a 2x4 sized piece of wood. The 5 floor slats looks roughly the same sized. So with that we get 20" wide, which would be a narrow but doable staircase. The landing width of the stairs in my SF apartment (which is new and to code) is 30". So not sure what code is, just a datapoint.