Various embodiments herein relate to methods, structures, tools, installation systems, etc. for retrofitting a new electrochromic window in a pre-existing window recess. In many cases, the new electrochromic window is installed parallel to a lite of a pre-existing window, with the resulting structure including the new electrochromic window, the pre-existing window, and a pocket that forms between them. Installation of a new electrochromic window in tandem with a pre-existing window results in many benefits including improved insulation (e.g., due to the presence of the additional air pocket(s) and lite(s)), improved climate control (e.g., due to the ability to control the amount of sunlight entering the building via the electrochromic window), and enhanced aesthetics.