A building system includes: an exterior mount; a wall panel; and an exterior attachment. The wall panel includes: a left column element carrying vertical loads through a left side of the wall panel abutting a right column element of an adjacent wall panel; an exterior surface extending across the left column element and cooperating with a second exterior surface of the adjacent wall panel to form an exterior façade; and a left channel section coupled to the left column element, recessed behind the exterior surface, cooperating with a right channel section of the adjacent wall panel to define an exterior mounting receptacle, facing outwardly from the wall panel, extending vertically between the wall panel and the adjacent wall panel, recessed behind the exterior façade, and configured to transiently retain the exterior mount. The exterior attachment: transiently installs on the exterior mount; and extends from the exterior façade.