This paper presents the results of a research project carried out in Spain that aims to develop a method that let employers to assess, during the design phase, the occupational health and safety costs of a specific construction project. This method classifies costs in four categories: insurance costs, prevention costs, accident costs, and recovery of costs. Labor accident data were obtained from 1990 to 2007 for the entire Spanish construction industry, and these data were subsequently homogenized and exploited. A mathematical model was created for computing each cost category. This method allows employers and project managers to estimate aprioristically the cost incurred as a result of occupational health and safety during the project, based on tangible values such as the construction project budget or the work schedule, as well as statistical data. An application to this method in a case study illustrated that the occupational health and safety costs for that construction project came to approximately 5% of the total cost of the budget. © 2014, Korean Society of Civil Engineers and Springer-Verlag Berlin Heidelberg.