The first clear sign that a home belongs to a Jew is the mezuzah on the front door. While it is hard to imagine a case where the front door would not require a mezuzah, there are several halachik issues to consider.
Which door is the front door?
It’s not always obvious, and the answer to this question often has important ramifications for other doorways in the house.
Halacha defines the front door as the one that is most often used to enter the house. Nowadays, that is not always the door that faces the street. Instead, it is often the garage door or a side door. While one would not make a bracha upon affixing a mezuzah to a garage door that is a building separate from the house, an attached garage would be considered the front door if it is the entrance used most often. Therefore, a bracha should be recited when putting up such a mezuzah.
Aside from when to say a bracha, identifying the front door is important as it defines on which side of the door to place the mezuzahs inside the house. A doorway from the outside of a house will always have the mezuzah on the right side as one walks into the house. Things get more complicated inside the house since the placement of the mezuzah is based on the most common flow of traffic (Igros Moshe YD 4:43:4 addressed to Rav Aharon Felder, zt”l).
For example, a door leading down to the basement should generally have a mezuzah affixed on the right side as you go down the stairs. However, when the garage is in the basement, the normal flow of traffic is likely to enter the home through the garage door and walk up the basement steps into the rest of the home. In that case the mezuzah on the door to the basement should be on the right side as one comes up the stairs and onto the first floor.
Should one make a bracha when affixing a mezuzah to the ‘front’ door?
When putting up mezuzahs for an entire house the standard practice is to make a bracha on the first mezuzah that one affixes and then continue to affix the rest of the mezuzahs throughout the house based on that initial bracha that was made.
If the front door opens to a large room, it requires a bracha like any other large room. However, if the front door opens to a smaller area, like a coat room or a mud room that is less than four square amos (approximately 7569 square inches) then that door may not technically require a mezuzah and one wouldn’t make a bracha when it is put up. Many authorities suggest that such an area may qualify for a mezuzah under the category of being a gatehouse. However, due to the uncertainty surrounding this type of room, it is best not to make a bracha on this mezuzah.
Because of uncertainty regarding the level of obligation of the front door it is often best to make a bracha on the mezuzah of a bedroom, which certainly has a Torah obligation, and then continue to affix the rest of the mezuzahs in the house.
Does a covered porch in front of a front door require a mezuzah?
A small room in front of the house is known in the Talmud (Menachos 33b) as a gatehouse. If it has a doorway for entry it is obligated to have a mezuzah. However, if an entrance is simply comprised of pillars that are designed to hold up the roof, it does not need a mezuzah.
A large porch (more than four square amos) in front of a house will often qualify as a gatehouse and require a mezuzah. Many authorities rule that even a space smaller than four square amos requires a mezuzah as the minimum size does not apply to a gatehouse.