The Yerushalmi writes that if you make a sukkah for a neighbor you should recite a bracha “la’ason sukkah lishmo”. We don’t find such a bracha in the Bavli, and it seems odd – why the need for a special bracha when ding a mitzvah for someone else? Tosfos (46a) references this Yerushalmi and apparently this idea caused the Maharasha discmofort as well because he changes the girsa to “la'asos sukkah lishma”, and suggests the Yerushalmi holds like Tanaim who hold sukkah must be built lishma.
I think the Yerushalmi can be understood k’pshuto. If you build a sukkah for yourself, the bracha is on the kiyum mitzvah of sukkah (please let's not go down the road of whether it is a hechsher mitzvah or not). But if you build for someone else you are not being mekayeim the mitzvah of sukkah – you are being mekayeim the mitzvah of arvus! On that mitzvah there is a separate bracha of “lishmo”.
Since I’m out on a limb anyway, let me throw this out as well. There is a famous story of a guest visiting R’ Chaim Ozer on sukkos when R’ Chaim Ozer was ill, and R’ Chaim Ozer insisted on coming to sit in sukkah with the guest. When asked why he did so when a choleh is patur from sukkah, R’ C. O. replied that a choleh is not patur from hachnasas orchim. I saw in Shu”t Az Nidbaru that R’ Binyamin Zilber was asked whether since R' Chaim Ozer was mechuyav to entertain his guest in the sukkah m’din hachnasas orchim does that in turn remove the ptur choleh from sukkah and leaving the original chiyuv of sukkah in place. Sounds like a wild idea, but it triggered the thought of a pshat that I ask mechila for in advance if you don't like it. Someone who builds a sukkah for a neighbor is mekayeim two birds with one stone: the mitzvah of sukkah and chessed for a friend. Ordinarily an act of chessed is not mechayeiv a bracha, but when paired with the already existing kiyum of sukkah, perhaps the Yerushalmi holds that the bracha is recited over both kiyumim – “la’asos sukkah” on mitzvas sukkah, and “lishmo” on the act of chessed. (Eve if you don't like it, at least give me points for creativity : )