Energy performance of the buildings can be evaluated by measuring energy consumptions or performing simple or detailed simulation methods. In this study, IZTECH Administrative Building which is a university building, is selected as case study and energy audit is performed to evaluate energy performance of the building. Indoor conditions are recorded with dataloggers and outdoor climatic data is taken from IZTECH Meteorological Station. Fuel consumption is tracked by two flowmeters and electricity consumption is measured with three power analyzer connected to heating, cooling and lighting and general use electricity meters. Energy consumption of the building is simulated by three different methods which are TS 825 (static), CIBSE Admittance (simple dynamic) and ASHRAE Heat Balance (full dynamic) methods. Sensitivity of these methods is tested by comparing energy consumption measurements and simulations and performance improving measures are proposed. Also a hypothetical no-HVAC case is simulated. ASHRAE Heat Balance Method is determined as the most accurate model compared to measurement results and performance improving measures are simulated with this method, reduction in energy consumption and greenhouse gas emissions are observed.