Public and private sectors are concerned with controlling the undesirable environmental impact of the new construction and the renovation of buildings. The building industry accounts for 32% of global energy consumption, 19% of energy-related CO₂ emissions, 25% of global water, and 40% of global resources depletion. The operational and maintenance phase of buildings accounts for 70% to 90% of the overall impact on environment. The literature review revealed many rating systems that were developed during the last few decades to assess the sustainability of buildings based on economic, social and environmental criteria. However, they represented local industries and none of them proposed a tool to select the best economic rehabilitation alternatives to upgrade building sustainability. Therefore, the main goal of this research is to establish an integrated sustainability rating and rehabilitation selection tool for buildings to fulfil the following objectives: 1) identify and study sustainability assessment attributes, 2) develop a sustainability assessment model for buildings, 3) build a sustainability scale and 4) establish a sustainability-based rehabilitation model for existing buildings. The research utilized several modelling techniques, such as fuzzy TOPSIS technique to determine the weight of each assessment attribute, simulation to determine the energy consumption, BIM-based model to assess building sustainability and the artificial immune system (AIS) to develop the sustainability-based rehabilitation model. Several types of data were collected and used to develop the aforementioned models utilizing questionnaires, case studies and interviews with facility managers from Canada and Egypt. 
The results showed the significant influence of the regional variations on both the weights of the sustainability attributes and the total sustainability assessment. By using a scale from zero to one, Canada showed the highest weights in energy, indoor environmental quality (IEQ) and building management criteria with values of 0.220, 0.167, and 0.156, respectively, whereas in Egypt, energy, site and water use criteria possessed the highest weights with values 0.2, 0.191 and 0.169, respectively. The sustainability-based rehabilitation model was implemented with a case study in Canada in which the results showed the capability of the developed optimization model to determine several optimal or near optimal alternatives to upgrade the sustainability with minimal life cycle cost (LCC). The developed tool was validated by experts through an interview and questionnaires showing the potential application of the tool to existing buildings. The assessment model was also validated through a comparative analysis between the proposed model and other well-known sustainability rating tools, which showed good potential. Sensitivity analysis was conducted showing the impact of the weight variation on the sustainability assessment. The research concluded the importance of introducing a multi-level weighting scheme in the assessment to reflect regional variations. 
The main contribution of the present research is to provide decision-makers with a two-tier tool that 1) determines the current sustainability of buildings and highlights the weak areas that require more attention, and 2) proposes various rehabilitation alternatives that upgrades the sustainability of the building with minimal LCC utilizing multi-objective optimization. The research also contributes to the body of knowledge by developing an integrated sustainability assessment and rehabilitation framework as a step towards establishing a comprehensive global sustainability-assessment tool.