Abstract The world is currently confronted with numerous challenges related to energy consumption and the limited availability of resources. Buildings, as significant consumers of these resources play a crucial role in addressing these issues. Efforts are underway to enhance the energy efficiency of buildings across various sectors, including mosques. Retrofitting mosque buildings is a key intervention that not only improves their sustainability but also sets them as exemplary models due to their spiritual and social significance. This study aims to explore various strategies for retrofitting existing mosque buildings to reduce energy consumption and increase sustainability, using the hot climatic conditions and the urban context of Saudi Arabia as a case study. The research will involve numerical simulations of several retrofitting strategies, such as installing external thermal insulation for roofs and walls, upgrading windows, and optimizing natural lighting. It will utilize DesignBuilder as a modelling tool to investigate the impact of these strategies on reducing cooling and lighting energy consumption in a set of simulation cases. The goal is to develop a set of design recommendations for modern mosques that promote sustainability and energy efficiency. Keywords: Buildings, energy efficiency; mosque; retrofitting; Saudi Arabia.