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There is a mitzva to live in a sukka during the seven days of Sukkot, as we read, “You shall live in booths (sukkot) seven days” (Leviticus 23:42).
What is the definition of a sukka? A temporary dwelling. Thus, a sukka must meet both of these conditions: 1) it must be habitable and 2) it must be temporary.
Therefore, a sukka is invalid if it is less than ten tefaĥim (30 inches or 76 cm) high or less than seven tefaĥim (21 inches or about 53 cm) wide. Given that not even one person can squeeze into it to sit and eat, it cannot be considered a dwelling at all. On the other hand, a sukka is invalid if its roof is made of boards like the roof of a regular home since this roofing is permanent.
The roofing must be of a temporary nature, but the walls may be permanent. Therefore, if people build a room in their home with a retractable roof, they may retract the roof before Sukkot, replace it with sekhakh (section 9 below), and use the room as a sukka.
Since a sukka is a temporary dwelling, it may be built on top of a car or on a boat, and may be used while traveling.
A sukka with three walls is kosher (valid). However, it is preferable to build a sukka with four complete walls and a door that can be closed, so the sukka will be comfortable and also protect people from the sun and wind. If necessary, the walls may be made of plastic or glass (even though the sun will penetrate), thin cloth (even though the cold will penetrate), or fence netting (even though the wind will penetrate).
The sukka must be under the open sky, so the sekhakh and nothing else 'protects' those who are in it. Therefore, a sukka built under a regular roof or under a tree is invalid.
