Sustainable design is believed to stand on the opposite side of heritage conservation. This view is supported by the fact that sustainable design requires invasive measures to implement new technologies and treatments that challenge the principle of minimum intervention in heritage conservation. Another point of view sees heritage conservation as an already act of sustainable development that protects and preserves social and cultural resources such as heritage buildings and their intangible values. On the other hand, research and practice have proven that heritage buildings can be the subjects of sustainable design projects that achieve outstanding measures of sustainability and energy efficiency while not compromising the authenticity of the heritage value of the building. This sustainable conservation reaches its peak in adaptive-reuse projects of heritage buildings as reusing the building guarantees its ongoing maintenance and promotes its social, cultural and economic values to society, while giving it the ability to withstand modern users’ comfort and energy efficiency standards. This research presents a case study of the adaptive-reuse project of Villa Antoniadis in Alexandria; a heritage building built in the mid-nineteenth century and in the process of a major adaptive-reuse project. The history and significance of the building will be studied as well as the conservation values of the current project, then some proposals for interventions that could achieve more energy efficiency for the project while conserving the building are discussed. The research included a simulation of the building, using building energy modelling software for the current adaptive-reuse project as a base case, and the hypothetical application of different proposed sustainable interventions such as thermal insulation, double glazing, shading, lighting control, natural ventilation, and photovoltaic energy generation, where the energy savings potentials for each proposed intervention were studied. The simulation proved a possible reduction of 36.5% in the cooling, heating and lighting energy consumption as well as generated 74.7% of the energy required for cooling, heating and lighting from renewable energy sources.