http://revach.net/article.php?id=943

Section: Halacha   Category: Succos
Aruch L'Ner: Building a Succah on Chol Ha'mo'ed

Building is not permitted on Chol HaMo'ed.  If someone does not have a succah is he permitted to build one on Chol HaMo'ed because it is a Melachah that is needed for Yom Tov. Is it permissible to build a succah on Chol HaMo'ed if one has a succah, but he needs a bigger one because he is making a bris and there are many people joining him in the succah?

The Bikurei Yakov (Aruch l‘Ner) says that the Melachah of building the succah is permitted in this case because it is regarded as something that is needed for the Moed. Even though normally a Melachah that is needed for the Moed is only permitted if it is done in privacy, in this case it is apparent and obvious that the building of the succah is for the purpose of the Moed and therefore it is permitted to build the succah publicly.