The time required for construction processes is closely related to the complexity of the project and its specific nature (variable impact factors). A new scheduling method with a probabilistic approach has been developed – Probabilistic Time Couplings Method III (PTCM III). PTCM III is based on the assumptions of Time Couplings Method III (TCM III), which has been improved by using the predictive Multivariate Method of Statistical Models (MMSM) and construction process standard deviations. PTCM III enables the time-cost optimisation of a construction project while taking into account the risks and uncertainties of construction processes. The priority of the method is to minimise the execution time of the project, with the possibility of downtime of work sectors or workers. The paper presents presents the formulas and calculation methodology, based on algorithmisation. In order to best present the capabilities and effects of PTCM III, a case study was carried out in the paper, in which the results of the calculations were compared with other variants of time coupling methods in probabilistic terms. The result of the PTCM III calculations is a range of the duration of a construction project together with the probability of its realisation. The most pessimistic completion time was calculated to be 412.7 working days, the most optimistic was calculated to be 388.7 working days and the 50% completion probability was determined to be 400.7 working days. This allows for more flexible construction scheduling and the adoption of lead times based on the contractor's capabilities.