Abstract Daylight is often considered an effective resource for saving energy and optical comfort in a building. People at workplaces expect appropriate daylight. The quality and amount of the light that enters a building depends on the internal and external features of the structure. Offices in Tehran are in abundance and are among the largest consumers of electricity. Thus, proper office design and suitable lighting can positively affect the financial situation and health of the employees. Placing an atrium in the center of the open offices, Kaveh Glass Office Building in Tehran features a comfortable atmosphere. Accordingly, this paper is focusing on assessing the daylight in this office building through changing the atrium structure. The atrium is a key component of a sustainable environmental approach to architectural design. The most crucial aspects of designing a building in a warm climate are solar irradiance and wind flow, and atrium defined as an element to control and benefit from them. Changing the architectural aspect of atrium had a crucial effect on the illuminance. This study was an attempt to obtain the optimum illuminance by validation and simulation of the illuminance data using such outputs as daylight autonomy (DA) or useful daylight illuminance (UDI) at different ratios of atrium height (H), width (W), and Length (L). When the well index is approximately 1 and the width to height ratio is approximately 0.5 in this building, the result is the maximum UDI and DLA.