Before setting out to buy mezuzahs you first need to calculate the quantitiy needed. In most cases, any doorframe that leads to a room requires a mezuzah. A room for this matter is a space not smaller than 2 by 2 meters, bordered by four walls. It can be a corridor, hallway, or even a storage room. Doorframes without a door (like passages between rooms, or a staircase also need mezuzahs.

Toilets, bathrooms, and temporary rooms (like a sukkah) do not need a mezuzah. A storage room which is located outside the house and is not smaller than two by two meters needs a mezuzah. If the objects stored in the room are useful and people frequently enter to take them out and use them the mezuzah should be installed with a blessing. But if the storage room is very rarely opened the mezuzah should be installed without a blessing.

Gates of gardens and entrances to cities and countries need a mezuzah.

The doorframe of a guard's hut at the entrance of a house does not need a mezuzah because it is not intended for residence. Nor do an exterior staircase or an animal pen.

A public bathhouse doesn't need a mezuza but a dairy barn, a chicken coop, a granary and a wine cellar do need. But some claim they don't.

A house or a garden where a non Jew is also living does not need a mezuzah.

Horizontal doors set into the floor leading to a cellar do not need a mezuzah, only vertical, standing doors.

A house rented overseas, during a period of less than 30 days, does not need a mezuzah.

Some of the above information was translated and simplified from the The Shulchan 'Aruk

Previous Article: The Mitzvah of the Mezuzah