A succa in which the sichach [ceiling] is higher then 20 amos is posul [invalid]. R’ Yehuda says it is good.

If the sichach is not higher then 10 tefachim [handbreaths], or it has more holes then material, and there are not three walls, the succa is posul.

Sichach that was put on  before Yom Tov, 30 days beforehand Bais Shamai says is posul, Bais Hillel says it is kosher. If made for Yom Tov it is certainly kosher.

Succa 1:2

If you made a succa under a tree, it is posul.

If you made one succa on top of another, the bottom one is posul. R’ Yehuda says if the bottom sichach cannot hold what is needed for the top succa the bottom succa is kosher.