Within the maintenance activities of non-residential real estate, there is a substantial potential to implement energy efficiency measures leading to the reduction of energy consumption. A focus on the optimization of energy usage can contribute to lowering overall maintenance and energy costs, preserving the technical performance of a building, and realizing energy saving objectives. Therefore a dynamic assessment tool to support an optimal energy usage in a non-residential building was developed. The tool is based on a system dynamic model that has sensitive variables for: two potential maintenance scenarios, external factors, and case specific conditions. In addition, by introducing a case study of Nijmegen City Hall the assessment tool has been verified.