The total amount of cost overruns for any construction project can be fully determined once the project is completed. Estimating the amount of cost overruns at different stages of ongoing construction projects is important for project success. There is, however, a dearth of research for this exercise. This article reports the results of an investigative study on cost overruns for ongoing building projects in Abuja. The quantitative technique was adopted in this study. The investigation included ongoing building-construction projects within Abuja, from which a sample of 30 building projects (public and private) was purposively selected (project value of ZAR100 million and above). The data were sourced from the archival records (drawings, bills of quantities, project progress reports, and specifications) on the issues relating to the costs and duration of building projects. The data were analysed using descriptive (percentages) and inferential methods. The results revealed that the percentage of cost overruns ranged from a minimum of 5.56% with 90% project completion, and within 88% of the estimated time limit, to a maximum of 216.08% with merely 5% project completion, and within 8.3% of the estimated time limit. The entire projects had average cost overruns of 44.46%, with an average project completion of 52.4%, and within 91.4% of the average estimated time limit. Based on these findings, it can be concluded that continuous investigation into, and analyses of cost overruns at stages of building projects would encourage professionals to apply the best mitigation measures, in order to achieve a significant reduction in the total cost overrun at the completion of a project. Construction professionals should be well informed of these consequences (cost overruns) at an early stage, in order to evaluate the extent to which these consequences could be minimised.