An adjustable support system has a base bracket member, a clip member, and an outer bracket. The base bracket member has a first end wall that overlays a substrate, a body wall extending outwardly therefrom and a second end wall opposite the first end wall. The clip member has a base bracket member coupling, and a projecting arm. The base bracket member coupling is attached to the first end wall and overlies a portion of the body wall. The projecting arm is spaceable apart from the body wall. The outer bracket has a spanning wall with inner and outer sides. An outer end wall extends from the outer side. The spanning wall is slidably positionable between the body wall of the base bracket member and the at least one clip member, to sandwich the same. Wall assemblies and methods are likewise disclosed.