Halakha Imamat 23:42: Mishneh Torah, Shofar, Sukkah and Lulav, Kitzur Shulchan Arukh, Mishneh Torah, Positive Mitzvot & Sefer HaChinukh

[In a case of] a sukkah on top of a sukkah: The bottom one is disqualified — like one who made a sukkah inside a house — but the upper one is fit. To what are these words — that the bottom one is disqualified — applicable? When the height of the space of the higher one is ten handbreadths or more, and the roof of the bottom one is capable of holding pillows and bed covers from the top one — even if [only] with strain. But if the height of the higher one is [less than] ten, or the bottom one is not capable of holding pillows and bed covers from the top one — even if [only] with strain — (even) the bottom one is fit. And that is when the height of both of them (together) is not more than twenty ells. For the bottom one is made permissible by the skhakh of the top one.

A borrowed sukkah — as well a stolen sukkah — is fit. How is that? If one assaulted his fellow and expelled him from his sukkah, stole it and sat in it — he has fulfilled [his obligation]. For land cannot be stolen (such that he is ultimately only borrowing the sukkah). And [even] if he stole wood and made a sukkah from it, he has fulfilled [his obligation]. For it is an ordinance of the Sages that the owner of the wood is only [entitled to] the worth of the wood alone. And even if he stole boards and placed them there — and did not connect them and did not change anything about them — he has fulfilled [his obligation. In a case of] one who makes his sukkah in the public domain: It is fit.

Sick people and those serving them are exempt from the sukkah — and not just a sick person in danger, but even one whose head feels unwell or his eye feels unwell. One who is indisposed is exempt from the sukkah — he, but not those serving him. And what is [the criteria by which one is determined to be] indisposed? That is one who is not able to sleep in the sukkah because of the wind, or because of the flies and the fleas and that which is similar to them or because of the smell.