The paper deals with the optimization of concrete structure in terms of environmental impacts, durability, and cost. The issue of environmental impacts has been increasingly discussed in recent years, and demands are placed on the construction industry to reduce the negative effects on the environment. The software tool was developed within previous work and enables multi-criteria optimization of the reinforced concrete slab. As part of the optimization process, several environmental impacts are considered, for example global warming, acidification and eutrophication of the environment or the raw materials depletion. The software tool allows the user to set the weights of particular environmental impacts. Furthermore, the user can decide whether to include the impact of materials transport in the optimization process. This article focuses on the analysis of the influence of these user-defined parameters on the result of the optimization process. A sensitivity analysis is performed in this study and the dependence of the result on the user-defined parameters is evaluated. Based on the obtained results, recommendations for further research are made.