Many of the production facilities and technologies operated on the Norwegian Continental Shelf are getting older and need to be modified to maintain the production performance at the desirable level. Each modification project is unique, and therefore needs to be evaluated and verified with consideration to all specifications. However, the oil and gas industry experience shows that the execution of modification projects within predefined time and cost are rather an exception, than a rule. In this paper we identify and discuss some of the factors that cause time and cost overruns in offshore facility modification projects. Furthermore, we discuss alternative cost methods to improve the quality and the accuracy of costs and time assessments.