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Thank you for knocking on our door. Kiss the Mezuzah and step right in.
Does a Walk-in Closet Need a Mezuzah?
Q: When moving into our new home, we had thought that we had enough mezuzot for the rooms, but we hadn’t considered the walk-in closets — our old home didn’t have any. I’ve seen that some of my friends have mezuzot on their closets and some don’t. Are there any clear-cut rules?
A: A walk-in closet that is four by four amot requires a mezuzah on the right side of one entering it. If its area is sixteen amot sq., but it is long and narrow (e.g. two by eight amot), a mezuzah is placed there without a berachah.
If a walk-in closet is less than sixteen amot sq.,some poskim suggest that it is praiseworthy to affix a mezuzah on the right of the closet as it opens into the room, without a berachah. However, in America many rabbonim exempt this small walk-in closet in accordance with the view of R. Moshe Feinstein.
Additionally, all would agree that when this small closet is generally accessed by standing outside of it and reaching in, it is exempt from a mezuzah even on the right side of its door leading to the big room. Since the doorway is not for entry and exit, it is not truly a “walk-in” closet even when it is large enough for a person to “walk into” it. 
If the closet is inside the bedroom of a married couple, the mezuzah should be enclosed with two coverings, one of which being opaque. Even if the mezuzah is placed on the frame inside a closed closet door, it is advisable to cover it with two covers in case the door is inadvertently left open during marital relations.
 Kuntres HaMezuzah (286:149) writes that if it is less than 7 tefachim wide, its obligation is extremely doubtful.
 Hagahos R. Akiva Eiger 286:13, as interpreted by Chazon Ish Y.D. 168:5; Parashah Sedurah 42. Cf. Chamudei Daniel, cited in Pitchei Teshuvah Y.D. 286:13.
 Igros Moshe Y.D. 1:181. See Inside Stam p. 214.
 Shevet HaLevi 3:103; Kuntres HaMezuzah 286:104; Chovas HaDar 4:27.
 Shach Y.D. 286:10; Sha’arei HaMezuzah 18:9.