Succah 1:1-2

Succah 1:1

A succah taller than 20 cubits (about 30 feet) is invalid; Rabbi Yehuda says that it is valid. A succah is also invalid if it is lower than 10 handbreadths (about 30 inches), does not have three walls, or has more sun than shade inside. Beis Shammai say that an old succah – meaning one erected more than 30 days prior to Succos – is invalid, but Beis Hillel say that it is valid. If a succah was built to perform the mitzvah on the holiday, it is valid even if it was erected immediately after the previous Succos ended.

Succah 1:2

A succah under a tree is no more valid than a succah inside a house. If one succah is built on top of another, the top succah is valid but the bottom one is invalid. Rabbi Yehuda says that if the top succah has no occupants, then the bottom succah is valid.