פירוש על סוכה 1:2

כאילו עשאה בתוך הבית – and it is invalid.

Introduction This mishnah deals with a person who makes a valid sukkah (we will learn more about what a valid sukkah is later) but then something is placed on top of his sukkah that does not count as valid skhakh. This causes the otherwise valid sukkah to be invalid.

תחתונה פסולה – for it has two [sets of] S’khakh/coverings of the festive booth, and the Biblical verse invalidates a Sukkah that is underneath [another] Sukkah.

One who makes his sukkah under a tree, it is as if he made it within the house. A tree while still attached to the ground cannot be used for skhakh, the roofing of the sukkah. Skhakh must come from a natural source, but it must be detached from the ground. Therefore, if one puts his sukkah underneath a tree it is invalid, just as it would be invalid if one built a sukkah inside a house with the ceiling as his roof.

אם אין דיורין בעליונה – which is not appropriate for dwelling, such as [if] the lower roof is unable to endure/sustain pillows and cushions of the upper [Sukkah]. But the First Tanna/teacher and Rabbi Yehuda did not disagree if it cannot endure it all, and in that, the entire world admits that the lower [Sukkah] is valid with the S’khakh/coverings of the festive booth of the upper [Sukkah], for its roof is not considered a roof, and it is not a Sukkah that is underneath a Sukkah. But if it is healthy and cable of receiving the pillows and cushions of the upper [Sukkah], the entire world does not argue that it is invalid. But they disagree when it can receive it in an emergency/when needed, such as the case where the roof of the lower [Sukkah] shakes and vibrates from the pillows and cushions of the upper [Sukkah], where the first Tanna holds that in this case it is called a Sukkah that is underneath a Sukkah and is invalid, and Rabbi Yehuda holds that since it other than in the case of an emergency cannot accept the pillows and cushions of the upper [Sukkah], it is not considered a roof, and this is not a Sukkah that is underneath a Sukkah. But the Halakha is not according to Rabbi Yehuda.

One [who makes] a sukkah on top of another sukkah, the upper one is valid but the lower is invalid. Rabbi Judah says: if there are no occupants in the upper one, the lower one is valid. If a person builds one sukkah on top of another, it turns out that the skhakh of the bottom sukkah is the floor of the top sukkah. Even if the skhakh meets all other halakhic requirements it is still invalid because the fact that someone is living above makes it again similar to a person who builds his sukkah inside a house. Rabbi Judah holds that if there is no one who is living in the upper one, than the bottom one is valid. The upper sukkah is not considered to be living quarters unless someone is actually living there.