Abstract: Methods, systems, and panels for retrofitting an existing exterior of a building. In one example, the method may include: (a) obtaining data about an exterior geometry of the existing exterior; (b) using the obtained data, defining a custom retrofit panel system for the exterior geometry; and (c) constructing the custom retrofit panel system including a plurality of panels, in which constructing a panel includes: (i) forming a panel frame by additive manufacturing, the panel frame defining interstitial spaces; (ii) at least partially filling the interstitial spaces with at least one filler; and (iii) in which the constructed panel includes a building facing surface configured to mate with the exterior geometry of the existing exterior.