There is a mitzva to reside in a sukka throughout the seven days of the Sukkot festival, as the Torah says, “You shall dwell in sukkot seven days; all citizens in Israel shall dwell in sukkot, in order that future generations may know that I made the Israelites dwell in sukkot when I brought them out of the land of Egypt, I the Lord your God” (Vayikra 23:42-43). Similarly, it states, “After the ingathering from your threshing floor and your vat, you shall hold the seven-day festival of Sukkot” (Devarim 16:13).
The sukka that we are commanded to live in during the festival is defined as a “dirat ara’i” – a “temporary residence” (Sukka 2a). Thus, a sukka must meet these two basic conditions: 1) it must be temporary; 2) it must be habitable. Therefore, if a sukka is less than 10 tefaḥim (c. 80 cm) tall or less than 7 tefaḥim (c. 56 cm) wide, it is invalid, because it is too cramped even for one person to sit in it and eat. Even if a sukka is very long, if it is less than 7 tefaḥim wide, it is invalid (Sukka 2a; MB 634:1).
Since a sukka is a temporary residence, it does not need four walls. It is sufficient for it to have two walls plus a tefaḥ of a third. This partial third wall must be within 3 tefaḥim of one of the other walls (as we explain below in section 6).
If a sukka is more than 20 amot (c. 9 meters) tall, it is invalid, because a sukka must be a temporary residence, while sekhakh placed at such a height needs the support of a permanent structure. Note, however, that the main expression of the sukka’s impermanence is the sekhakh; the walls may be permanent, as long as this is not necessary to support the sekhakh. Thus, one may retract the roof in the home and place sekhakh instead of a ceiling; since the sekhakh is less than 20 amot high, it does not need the support of a permanent structure.
A house with a wooden ceiling is invalid for use as a sukka, because sekhakh must be impermanent, whereas a ceiling is permanent. To make sure that people do not mistakenly permit wooden ceilings, the Sages ruled that lumber commonly used to make ceilings may not be used as sekhakh (as we will explain below in section 4).
Since a sukka is a temporary residence, it may be built on a wagon, motor vehicle, or boat, and it remains valid even during travel, as long as its walls and sekhakh can withstand an ordinary wind (SA 628:2; SHT ad loc. 11). As a temporary residence, a sukka does not require a mezuza (SA YD 286:11).
A sukka is invalid if its walls are unable to withstand an ordinary wind or if its sekhakh is made of leaves or greenery that will wither and fall during the course of the festival. In both of these cases, it is not even considered a temporary residence (SA 628:2; 629:12).