Shulchan Arukh Orach Chayim · Siman 628

English

The Laws of Building One Sukkah Under Another Sukkah. Containg 3 paragraphs: Regarding a sukkah that is underneath another sukkah, the top one is valid and the bottom one is invalid. This is so if the bottom one is able to receive pillows and mattresses of the upper one, even with difficulty, and there are ten tefachim between them. However, if there are not ten tefachim between them or there are ten tefachim between them but it is impossible for [the floor of] the upper [sukkah to support a person together with his] pillows and mattresses even with difficulty, the bottom one is valid if it is covered according to the law, even if the upper sukkah is higher than twenty tefachim. If it is not covered according to the halachah and valid due to the s'chach of the upper one, the s'chach of that upper sukkah cannot be higher than twenty amot off the ground.

English

If a sukkah is constructed upon a wagon or upon a boat, if it cannot withstand ordinary land winds it is invalid. However, if it can withstand ordinary land winds - even if it cannot withstand ordinary sea winds - it is valid.

English

[If a person] constructs his sukkah on a camel or on top of a tree, i.e., the treetop, it is valid. One may not, however, climb up to it on a festival, even a little bit on the tree and a little bit on something else, if [the sukkah] is constructed in such a manner that if the tree was removed, it would still stand on the other support without falling on Yom Tov. And if not, he may not climb up on it on Yom Tov.