The aim of this study is to explore the effects of energy efficient design parameters on the thermal-visual comfort conditions and the energy load of office buildings through a case study of one selected office building in ?stanbul. In this study, DIALUX and DESIGN BUILDER programs are used in order to simulate the case example under different design options. Different scenarios are developed in order to obtain an optimized office envelope from energy efficiency and comfort conditions points of view. The effects of changes in design parameters like orientation, transparency ratio and material selection of the building envelope are evaluated through the mentioned computer simulation tools. Several configurations are compared with the base case unit and discussion of the results is performed under the main subject of energy efficiency.