Question: Does one need to put a mezuza on an outside exit door. If so, which side would it go on?
Answer: R’ Shlomo Zalman Auerbach (Minchas Shlomo 2:97:22) writes that an emergency exit that can only be accessed from the inside and is only used in case of an emergency is not considered an entrance to the house and so does not require a mezuza. Similarly, the Kitzur Shulchan Aruch (11:1) writes that if one has a doorway that is only used on an occasional basis for loading things in, one doesn’t need to affix a mezuza to it.
If one’s patio door serves as another entrance to their house, then one affixes the mezuza to the right side of the doorway from the outside. There is a machlokes, however, as to which side of the door the mezuza goes on when such a door opens out onto an enclosed balcony or garden. The Shulchan Aruch (YD 289:2) writes that if one puts up the mezuza on the wrong side then they haven’t fulfilled the mitzva. R’ Moshe Feinstein (Igros Moshe YD 1:176) writes that when in doubt one mustn’t place a mezuza on each side (See Binyan Tzion 89).
The Chazon Ish (YD 168:5) writes that it is considered to be an entrance to the house and so goes on the right side from the outside.R’ Moshe Feinstein (Igros Moshe YD 1:181) and R’ Ovadia Yosef (Yechave Daas 4:51 quoting many other poskim,) however, write that as the balcony or garden is only accessible from the house, the mezuza must be placed on the right side from the inside. R’ Ovadia Yosef writes that if the balcony is smaller than four amos by four amos (3.7m2 according to R’ Chaim Naeh; 5.9m2 according to the Chazon Ish) then it doesn’t technically require a mezuza though one should ideally affix one (See Shearim Metzuyanim Behalacha 11:3).