הלכה אויף ישעיהו 4:6

A sukkah that does not have three panels is disqualified. [If] it had two full panels — one besides the other — like a type of gamma — one makes a panel that has more than a handbreadth in width and places it less than three [handbreadths] adjacent to one of the two [existing] panels, and that is enough for it. However, one must [also] make the outline of an entrance for it — since it does not have three full panels. And we have already explained in the Laws of Shabbat that the outline of an entrance mentioned in every place is even a reed from here and a reed from there, and a reed on top of them — even if it does not touch them.

Any sukkah that is made properly is fit — even though it is not made for the sake of the commandment. And that is so long as it was made for shade, like a hut for gentiles, a hut for animals and anything that is similar to them. But a sukkah formed on its own is disqualified — since it was not made for shade. And likewise [in a case of] one who dug out [sheaves] from a stack and made it into a sukkah: It is not a sukkah — since he did not gather this stack for shade. Hence if he made a cavity [in the stack] of a handbreadth across seven for the sake of a sukkah at the beginning, and afterwards dug out [more] to complete it to ten, it is fit. For its skhakh was made for the sake of shade.