When performing green remodeling of public buildings, illegal buildings are demolished, and these illegal buildings installed for user convenience incur demolition costs. However, there is a way to legalize illegal buildings to increase the installation of renewable energy while saving demolition costs. In this study, the cost of replacing an illegal canopy with solar panels and the effect of reducing the primary energy demand were analyzed. As a result, converting the demolition cost of illegal buildings to the expense of installing solar panels is expected to contribute to cost-effective green remodeling, maintenance of user convenience, and an improved supply of new and renewable energy.