This study assessed the causes of cost overruns in public building projects in Uganda. A mixed design combined a relative importance ranking of ten perceived causes, based on 120 simulated practitioner responses, with a quantitative analysis of 140 simulated completed projects. Descriptive statistics, Pearson correlations, multiple linear regression and binary logistic regression were applied, with full diagnostic testing. Delayed payment of certificates, incomplete design at tender and variation orders were ranked as the three most important causes. The mean cost overrun in the simulated sample was 26.0 percent of the contract sum. The linear model explained 65.1 percent of the variance in overruns, with schedule delay, variation orders, incomplete design, delayed payment and delayed site possession increasing overruns and contractor experience reducing them. In the logistic model, incomplete design raised the odds of a substantial overrun 3.31 times. The study concluded that overruns originated mainly in weaknesses in project preparation, client-side payment performance and change management, and it recommended complete designs before tendering, timely payment and strict control of variations. All figures were simulated for illustration and do not describe real projects.