Question: A door into a closed garden normally has the Mezuzah on the right side on entering the garden (as with a Mirpeset). If there are multiple doors into the same closed garden all opening outwards to the garden and one other door opening from the same garden into the house, what is the placement? Answer: á"ä
There is actually a difference of opinion of the Gedolim of our time if exiting the house to a garden or "Mirpeset"=terrace if the mezuza should be placed on the right upon exiting the home or upon on the right upon entering the home from the mirpeset. The Chazon Ish, Shevet Halevi, and Rav Elyashiv all zt"l, held the latter opinion and many in Bnei Brak follow that custom While Rav Moshe Feinstein, Rav Ovadia Yoseph, and others zt"l, held the former opinion of placing on the right upon exiting the home. According to Chovat Hadar, this is the prevalent custom. Furthermore, while placing a mezuza for a Mirpeset, in should be included in the Bracha for the rest of the house and not to say a bracha for the porch alone.
It is difficult for me to visualize what you are describing and therefore, I think it is best to have a Rabbi come over and take a look or to show the Rabbi a few detailed photos so you can get the correct answer.
All the best