**5. Conclusions**

The phenomenon of exceeding planned investment costs is often encountered in the construction industry, and the determination of the risk associated with it may be of key importance for achieving the objectives of the project. This paper discusses a cost overrun risk prediction model, the development of which was based on the fuzzy inference model of Mamdani. The model input variables include the following: share of element costs in the building costs (SE), predicted changes in the number of works (WC), and expected changes in the unit price (PC). The basic problem is to adjust the shape of the fuzzy sets for a given input SE to the type of building object. The paper proposes a shape for cubature buildings (residential and office ones), highways and expressways, and sports fields.

In order to check the correctness of the assumption made of the rule database, result diagrams were generated for the relationships between the variable R and the input variables of individual types of buildings. The obtained results confirm the correctness of the assumptions. With an increase in input variables, the value of the risk level of exceeding the costs increases naturally and smoothly. The results prove that the input variable SE, adjusted individually to the model for each type of construction object, is crucial in the context of influencing the output value. The lower the membership for the values of the arguments of the X1 universe domain for the linguistic terms "average" and "high" of the SE variable, the more the resulting value of the risk of construction investment cost overrun (R) increases for the arguments of the X1 variable universe with smaller values.

The model requires further research, both in terms of the input data taken into account and the diversity of the analyzed construction projects. Further testing of the model on actual construction projects will confirm its usefulness in determining the risk of cost overruns.

**Author Contributions:** E.P. carried out a review of the literature concerning the introduction part. E.P. and D.W. described all assumptions of the cost overrun risk prediction model. D.W. prepared all figures and tables. E.P. and D.W. discussed the results, drew conclusions. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
