A borrowed Sukkah:[1]
One fulfills his obligation with a borrowed Sukkah.[2]
[1] Admur 637:3; Sukkah 27b
[2] The reason: Although the Torah states [Devarim 16:13] “Chag Hasukkos you shall make for yourselves” which means that one must own the Sukkah hence excluding the Sukkah of a friend, nevertheless one fulfills his obligation with a borrowed Sukkah. The reason for this is because when one enters his friends Sukkah with permission it is considered as if it is his, as the words “make for yourself” is only coming to exclude a stolen Sukkah, as will be explained in 637:5. [Admur ibid; This means to say that the verse does not require one to actually legally own the Sukkah but rather that one own the right to dwell in the Sukkah just like as if it was his. Thus, if borrowed it is valid as one receives the legal right to dwell in it just as if it were his. However, a stolen Sukkah one has no dwelling rights and it is hence invalid.]