Energy efficiency at the city level is mainly dependent on buildings and requires sustainably designed strategies. In this research, an energy index that leads to a classified measurement of sustainability at a building level is defined and evaluated based on a selected building in Istanbul. Sensitivity analysis is assisted by determining the precision of the outcome when certain interventions are applied to improve the building energy efficiency. Each intervention's energy consumption is evaluated based on the overall building energy consumption, which includes all building related energy consuming parameters. There are 6 different scenarios which are composed of various combinations of 8 interventions affecting 7 energy consuming parameters. A matrix is prepared for conducting sensitivity analysis with the particular effects of the applicable interventions. Furthermore, specific values are evaluated to find their sensitivity to affect the building energy performance. The most important improvements among all interventions are automation and monitoring with an energy consumption reduction of 26.07%, ground source heat pump at 16.43% and light emitting diode lighting with sensors at 9.26%. These interventions are also remarked as highly sensitive. As a result, the prepared matrix directly describes the effect of variables in the system numerically, which also assists to determine each intervention's sensitivity levels.