In this final paper, the topic of illegal construction and the procedure of legalization are elaborated, the Law on Treatment of Illegally Constructed Buildings is explained, and the procedure of legalization of illegal construction is presented, which documentation is required and how to get a solution about the derived condition. The costs that are present in the legalization process itself are described, as well as the costs incurred by the parties in the legalization process itself and after it. From the read literature on the topic and previous knowledge of the author, since he performed the job that were in one part related to the legalization process, the review and reflections of the same are permeated through the final paper.