Space geometry design is the first stage of proper daylight design. The basic ideas of form and space design should provide the best conditions for daylight. This article investigates the effect of living room proportions on the amount of interior lighting received in Tehran's residential buildings. In the first step, the information from literature related to daylight was studied. In the second step, a checklist has been compiled to examine the space based on daylight from the classification and summarization of the information obtained from the research background. Based on this theoretical framework, one of the houses in Tehran with a common plan was selected as a case study according to daylight design indexes to achieve the goal. In the third step, the problem was determined, and then the solution was provided by simulating using applied models and software. The modeling process and changing the geometric variables were performed parametrically with Grasshopper software, and daylight simulation was performed by Radiance and Daysim software. Thus, the intensity of annual illumination is examined in the current situation to optimize the illumination conditions; the window of this space is optimized by maintaining the optimum ratio of this window, the optimum space proportions obtained. The survey has shown that in optimal condition, the ratio of length to depth in the living room is 1.54 to 1.91, and the WFR ratio is 9% to 22%. Thus with the values obtained, the living room will have the best thermal and lighting performance during the year. In addition, with the optimal percentage of the window to the floor, appropriate shading device, and optimal dimensions of the room, this space can provide suitable light for various daily activities in a multi-purpose living space. پرونده مقاله