The diploma thesis titled »Investment analysis of deviation between projects and execution of sewerage systems disposal« researches preparation of the investment documentation with investments in systems of disposal and wastewater treatment and problems or imperfections that may occur during the process. The first part of the diploma thesis researches investments in general, their division, and different sorts of investments. The content of the investment documentation, its division, and demands which legislation claims for the area of preparation have been accurately described. The second part of the diploma thesis researches the disposal systems and wastewater treatment. The most important documents have been described which define rules for planning, building, managing, and maintenance of the systems and the needed infrastructure which has to be assured at different areas until the end of the year 2017 by the Republic of Slovenia. The third part of the diploma thesis includes an analysis of the investment documentation of a real project which was executed at the area of disposal and wastewater treatment in the past. The analysis has focused on discovering the mistakes or imperfections that may have occurred during the preparation of the investment documentation. While I was doing this analysis, I discovered how difficult it is to get investment documents of real projects. This can be a warning signal, because these documents are basic framework of using the public funds, and because of this they should be publicly available and very transparent. A part of the diploma thesis is also a cost model which has helped to assess the expenses of sewerage system installation, therefore, expenses which have been ineligibly left out of the investment documentation.