Most of the time we use them interchangeably to mean stairs. However, if we mean the ladder, we would most likely say "hagdan" and if we are describing a house in terms of its components, we would most likely say "hagdanan". You need not worry about which one to use specifically though, because you would be understood either way.

Hagdan(stairs/ladder) is the steps where we use to go up and down while hagdanan (stairway) refers to the whole model of the stairs and the area where the stairs is positioned or located.

salamat