A common misconception is that only the main entrance to the home requires a mezuzah. Of course, it is better to have one mezuzah on the front door than no mezuzahs, and if you only have one mezuzah, it should be placed on the home's main entrance. However, to properly fulfill the mitzvah, every room in the house or office should have its own mezuzah.
All rooms that are more than four cubits long and four cubits wide -- approximately 6.5 feet by 6.5 feet (or its equivalent of 37 square feet) are required to have a mezuzah -- as long as they have a full "doorway." A blessing is not recited, however, if the room does not extend at least 6.5 feet in each direction.
A doorway that requires a mezuzah is only one that that has two doorposts and a lintel connecting the doorposts on top. If these conditions do not exist, a competent Rabbi should be consulted to determine whether or not that entryway requires a mezuzah.
One does not affix a Mezuzah on bathrooms, shower rooms or the like.
Do garage doors, boiler rooms, attics, outdoor sheds, or crawl spaces require mezuzahs?
Yes, they do, provided that they comply with the required measurements (approximately 37 square feet ). At the entrance to a crawl space, the doorposts themselves must be at least 10 handbreadths high (approximately 32 inches).
An attic also requires a mezuzah unless it is entered via a "trapdoor" (a horizontal opening in the floor rather than a vertical opening in a wall).
What about porches or sun porches?
There is a difference of opinion whether the mezuzah is to be placed on the right side relative to a person entering the house, or on the right side relative to a person going out onto the porch.
Am I obligated to affix a mezuzah on the doorpost of a gate that leads to my backyard?
One has to affix a mezuzah only if there is a cross bar on top of that gate.
Do I need a mezuzah at my workplace?
Yes. In addition to private places of residence, mezuzahs should be affixed in businesses and stores without a blessing.
If you’re working for a non-Jew and you are renting space from him, a mezuzah should be affixed on your office door without a blessing.
However, if you are not renting the space, and there is the possibility that your office space could be relocated, then you would be exempt from affixing a mezuzah.
We have a non-Jewish live-in maid. Am I required to put a mezuzah on her door?
Yes, providing that upon your discretion, her room may be changed to another location in the house at any given time.
What if I never use that door?
Unused doors that are sealed closed (doors actually nailed to the door frame) are exempt from mezuzahs. An entrance that is blocked by furniture or a door that is usually kept locked requires a mezuzah.