For large projects of national importance, a spatial plan of national importance has to be prepared before siting can be conducted. This procedure is prescribed in Spatial planning Arrangements of National Importance Act, and also includes legislative acts and executive regulations related to other fields. Due to the complexity of the process, the master’s thesis includes a process diagram defining different activities crucial for preparing a spatial plan of national importance as well as the stakeholders involved in the procedure. The process diagram also includes all other (non-legislative) procedures of siting and environmental planning, in addition to activities connected with the preparation of project documentation and investment documentation, expert guidelines, implementation of project communication, etc. After defining the necessary activities in the process diagram for the spatial arrangements of national importance, the entire procedure was modelled using the IDEF0 diagram software. This was done to present the procedure more comprehensively (a tree diagram), thus making the entire process more comprehensible to all the stakeholders involved in the spatial plan of national importance. In addition to understanding the procedure, an investor also has to be familiar with the time scheduling and financial planning of the process. Therefore, we designed a time schedule with the Microsoft Project software where the estimated duration of each activity and the entire procedure of preparing a spatial plan of national importance are presented, as well as all incurring costs. However, the described approach does not include the possibility of additional (unexpected) influences on each activity (possible reasons an activity would take longer than planned), which is why the thesis also includes a comparative time schedule (together with the estimated costs) obtained with the Primavera software solution, enabling flexible deadlines for each activity and providing the probability of activity completion by a given date. A comparison of the results for time scheduling and financial planning obtained with Microsoft Project and Primavera solutions illustrates the influence of unexpected events on the duration of preparing a spatial plan of national importance, and the resulting differences in the estimated costs.