The building sector uses 40% of the primary energy worldwide. Energy demand in a non-residential existing building is rising due to higher comfort conditions, population growth and the enhancement of the building services. On the other hand, renewable energy technologies are every time more accessible for the built environment, especially the ones that use the sun as their primary energy source. The difference between that energy demand and that production of energy in existing buildings is causing a mismatch of energy that needs to be solved if problems such as an increase in the electricity bills, an oversized energy grid or the dependence on fossil fuels want to be avoided. Currently there is not research on how to reduce the mismatch in non-residential existing buildings; therefore, this research aims to be the beginning of the exploration of this topic. By matching the energy demand and the energy produced at any point of time within the building boundary, the building will be energy flat. During this research, the renovation of a case study building towards energy flatness was proposed. A three steps strategy was proposed to reduce energy demand, produce renewable energy and integrate a complementary energy system in existing buildings that want to be renovated towards a full energy balance and a renovation proposal of a case study building towards energy flatness. As a result, energy flatness in the renovation of non-residential existing buildings is limited to the extent of their physical and functional parameters which restrains the energy demand, production and distribution. Furthermore, the three steps strategy helped to reduce the energy mismatch and is relevant because the energy mismatch problem is going to be every time more visible in the built environment.