Eating in the Sukkah on Shemini Atzeret
Question
Is it permissible to eat the holiday meals in the Sukkah on Simchas Torah?
Answer
Shalom and joyous festivals!
It is forbidden to eat in the Sukkah on the eighth day, unless one makes a specific sign signifying that he is not doing so for the purpose of dwelling in the Sukkah.
The sign is made by removing a piece of the schach (Sukkah covering) the size of four by four handbreadths [forty by forty centimeters], and then one can sit in the Sukkah.
If one decided to sit in the Sukkah only once the festival already started, one should place a pot or something like that in it — things that are forbidden to bring into the Sukkah during the festival. By this one will reveal that he is not sitting there for the sake of fulfilling the commandment.
Source
Shulchan Aruch, section Orach Chaim, 666, paragraph 1