A succah that is more than twenty cubits high is invalid. R’ Yehudah validates it.  And [a succah] that is not ten handbreadths high, or [one] that does not have three walls, or [one] whose sun[ny area] is greater than its shade[d area], is invalid.  An old succah —- Beis Shammai invalidate it and Beis Hillel validate it. And what is [considered] an “old” succah? Whatever one built thirty days prior to the Festival. But if one built it specifically for the Festival, even [if he put it up] at the beginning of the year, it is valid.

If one builds his succah under a tree, it is as though he had built it inside the house.  If a succah [is built] atop another succah the upper one is valid, and the lower one is invalid. R’ Yehudah says: If there can be no tenancy in the upper one, [only] the lower one is valid.