Construction of reinforced concrete buildings on soils with unknown underground void locations may lead to building instability and tilting. The current study investigates the influence of the existence of underground voids on surface-reinforced concrete structures under gravity loads only. A numerical analysis using a finite element program (ANSYS) was performed to study the behavior of reinforced concrete buildings with different widths and heights. The buildings were rested on clay soil with a continuous circular void for a wide variety of void diameters and locations from the building's footing. Charts for settlements and differential settlements between footing edges for all cases were obtained to evaluate the effect of these parameters on the behavior of surface buildings. Based on the obtained results, equations for design were provided for the critical area underneath concrete structures affected by underground voids, according to the building size, void location, and diameter. These equations introduce simple tools to predict the critical void location within the variety of parameters that have been studied in this study.