A succah is kosher even if it was not constructed for the purpose of fulfilling the mitzvah. The Gemoroh discusses Succos of non-Jews, women, animals, Cutheans (a pseudo jewish sect), shepherds, fruit picker, guards, and field watchman. These are refered to by the title "ganbach and rakbash". All of these are kosher, as long as they meet the halachic requirements of a succah. Meaning that they are made to protect from the sun and heat (not just for privacy) (Tur). Another requirement is that it should not be made as a dwelling for the entire year, for that would render it his home of the entire year and not a succah. (From Rashi it seems that if it is a house that is just used as a summer residence, it would be kosher [Biur Halachah]) The reason for this prohibition is because the Torah tells us to dwell in a succah and not in a house where we live the entire year (Aruch Hashulchan section 2)2. Do we see any other new concepts in reference to "ganbach and rakbash"?
Yes the reason the Gemoroh cites the the first four "ganbach"; a succah of non-Jews, women, animals, and cuthean, in order to teach us the following. Even though they are not obligated in the mitzvah of succah nevertheless their succah is kosher.Next Chapter
The succos of rakbash shepherds, fruit pickers, guards and field watchmen, teach us that even though their succahs were not stationary but portable. shepherds, for example would travel from place to place looking for pasture, likewise fruit pickers when the figs dried out would uproot their succahs. Nevertheless these succahs are kosher.
Even a succah which has two faults for example a succah built by non-Jewish shepherds is also valid. (M.B. section 2, Shaar Hatziun section 6)
Even though the succahs of "ganbach and rakbash"are kosher a "renewal" should be performed (see chapter 12).