Between the ideal and reality lies the decisive world of the performance gap. This project is conducted within the framework of a Master Thesis at the Industrial Processes and Energy Systems Engineering (IPESE) laboratory of Ecole Polytechnique F´ed´erale de Lausanne (EPFL). The objective is to evaluate the energy performance gap, with a static approach, of the building sector and understand its impact on the global energy system in the context of energy transition. The challenge lies in the conservative assumptions regarding the thermal transmission coefficients and construction details of existing buildings in modelling tools. This study is based on two detailed surveys, one on swiss architectural elements and the other on new thermal transmission coefficient ranges. A methodology is designed and integrated to the existing optimization model: Renewable Energy Hub Optimizer (REHO) for different types of buildings, each having particular features. The idea is to integrate to the REHO model a static approach by varying the thermal transmission coefficients and develop a new method concerning the link between the thermal envelope and energy reference area. In result, the impact of the thermal envelope and the form factor of buildings on space heating requirements is researched. The investment and operation uncertainties resulting from the modelling gap are assessed. Finally, the static model developed in this study is compared with a regulatory approach and real on site data from a clustered neighbourhood in Geneva. The results show an improvement of 13,4 % with a change of method.