Q. Can I eat in the sukka while building it or when taking it apart after Yom Tov? Is that Baal Tosif?

A. Baal Tosif only applies when someone has intention to add days or situations that were not commanded by the Torah and treat them as if they are part of the mitzva, when they are not.

Rabbi A. Bartfeld as Revised by Horav Shlomo Miller Shlit”a