Achsadra: A structure with only three walls:
An external structure that only has three walls is called in Achsadra. An Achsadra which contains a completely open fourth wall which is used as the entrance does not require a Mezuzah on its side, as its sides are not considered to be doorposts, as they are there to support the other walls. [Thus the bus stops here in Israel do not require a Mezuzah. Exhibit A] If however there is an extension of a wall in the opened fourth wall area, then it is obligated to have a Mezuzah according to all. [Exhibit B]
Leads into a room with walls, such as the entrance to a building: If the Achsadra leads to another room which requires a Mezuzah, as is common with an Achsadra that is built in front of one’s front door of his house or building [exhibit C], some opinions rule that the entrance of the Achsadra is to have a Mezuzah.
Inner room with three walls: The above law of an Achsadra refers to an external three walled structure. However in a house or building, if an inner room is missing an entire fourth wall which is used as its entrance [exhibit D], then one needs to place a Mezuzah on it, as the turning sides of the two walls serve as the right and left doorpost.
 Michaber 286/6
 This is commonly found in front of the entrance of homes. See Pischeiy Shearim 286/95
 Kitzur Shulchan Aruch chapter 11/16. However from the Michaber is implied that even in such a case a Mezuzah is not required. The Misgeres Hashulchan in the Kitzur SH”A wonders why the Michaber did not mention this ruling.
 Taz 287/1 in name of Tur