In each country, the concept of housing exists in relationship between the ways people live, the local climate, the social and political factors that affect the evolution of architecture and the actual structure of the house. The economic developments of the countries and the political situations and regulations have all affected the rise of the residential market. In Greece, where the majority of the residential buildings were constructed between 1960 and 1990, residential buildings represent an important sector in the energy consumption and economic growth of the country. Global warming and changes in the climate system has an important impact in building stock. The European Union is concerned about the scale of the consequences and has encouraged energy efficiency in buildings by mandating the Energy Performance Building Directive for all European countries [1]. In my opinion, it is the responsibility of the architect to address the impact of existing residential buildings by retrofitting solutions with the aim of reducing the energy consumption and eventually improving the quality of life. Architects, in collaboration with energy experts, can work properly to provide effective solutions. With the guidance of the new directive, along with their personal knowledge and experience in energy efficiency they can help bring about increased energy building performance. In order to demonstrate how architects can implement such an energy retrofit plan, this thesis presents a short description of residential buildings constructed between 60's - 80'by presenting a typical building representing the current state of the Greek building stock, and by studying its energy consumption. An energy performance simulation of the building with different energy simulation software considers different scenarios and describes a way to reduce energy demand and increase comfort in these buildings. The primary software tools used in the study was TEE KENAK, developed by the Greek chamber in accordance with the European energy rating tools for the implementation of the Energy Performance Building Directive. Additional software tools, including ECOtect, and the "Energy Performance Calculator", developed by the department of High Performance Buildings at the Georgia Institute of Technology in order to verify the information provided by the actual energy bills and the TEE KENAK. The results presented in this study estimate the building retrofit and energy saving, making appropriate decisions in terms of energy conservation and improvement in the existing residential building considering energy cost savings, payback from retrofit investments, along with architectural design considerations.