Someone who did not build a succah before Yom Tov, whether by mistake or intentionally, would be permitted to build one on Chol Hamoed. He would not be penalized for not making it before Yom Tov. This would also be the case if his succah fell down during Yom Tov, or for a non-Jew who converted to Judaism during Succos. It would be permitted to build one even at the end of the last day of Chol Hamoed (Shulchan Aruch section 1 , M.B. section 1).2. Does this permission to build a succah on Chol Hamoed include doing types of work that are normally forbidden on Chol Hamoed, for example actual construction and tearing down?
There is an opinion which holds that the permission to build a succah on Chol Hamoed is only in reference to the making of the s'chach. This means placing the s'chach upon the succah frame which has been constructed prior to Yom Tov (Maimar Mordechai brought down in the Biur Halachah). However the Chofetz Chaim concludes in the Biur Halachah that it seems clear to him that it is permitted to construct an entire succah on Chol Hamoed.3. Is it permitted to use several succahs during Succos?
It is permitted to leave one succah and use another (Shulchan Aruch section 1). It would also be permitted to eat in one succah and sleep in another, for we do not require that one succah be used for the entire seven day period (M.B. section 3, 4).Next Chapter