Urban growth is susceptible to the effects of climate change, and those effects are anticipated to worsen in the years to come. Egypt experiences a roughly seven billion cubic meters of water shortfall each year. Egypt has issued GPRS (Green Pyramid Rating System) to measure and certify green buildings since its implementation in 2010, however, the application of GPRS is still limited as it assesses the building's sustainability performance and calculates the building credit ac-cording to water efficiency themes without supporting or recommending practices and criteria to achieve. This paper aims to create a water rating system for existing constructed buildings in Egypt and a computational code to relatively compare the existing sustainable practices, and finally to implement the rating system by identifying the essential sustainable practices and criteria. Three established global rating systems (LEED, BREEAM, CASBEE) and two local rating systems (GPRS, TARSHEED) were reviewed, and comprehensive water efficiency criteria were developed that include 15 factors grouped under 6 main categories: storm water management, reuse and recycling, management and operation, irrigation and landscaping, conservation, and monitoring systems. Then, a questionnaire was developed to assess the relative importance of each set in the criteria according to the 78 participants' answers that were analysed using Analytic Hierarchy Process (AHP). The main essential practices from the 15 to consider in existing buildings were processing water reduction, integrative water systems, and indoor efficient water use design. The results of this research could contribute to the update and development of the local sustainability rating systems (GPRS & Tarsheed) in terms of measures used to achieve water efficiency or the weights associated with each measure