Mezuzah on the entrance between a house and its backyard

Hi,

If a house has an enclosed backyard, such that the only entry to the backyard is from the house, then the backyard is viewed as a room of the house. Therefore, the mezuzah is placed on the right side of the entrance to the backyard, from the perspective of a person going from the house to the backyard.

(Rav Moshe Feinstein, Igrot Moshe Yoreh Deah 1:181)

Have a great day,
Mordechai