In this thesis we described law regulations linked to legalization of illegal built buildings in area of Republic of Croatia and entire process of legalization. We described the geodetic methods of measuring, as well as the procedure of making geodetic elaborate in purpose of legalization of a building are described. In the end, concrete examples of implementation in area of Istria county are shown. The analysis of solved legalization requests was shown, as was the dynamics of solving the requests for before mentioned county.