Coastal flooding is typically caused by combinations of extreme water-levels and large waves. Two extreme value methods, one univariate and the other multivariate, have been used for estimating the probability of coastal flooding at an existing flood defence structure and for aiding the design of a new structure. The properties of these two methods are compared in terms of extrapolation, sophistication and use of information for a range of extremal dependence structures. We find that, when applied to the assessment of the safety offered by an existing Dutch dike, the multivariate approach provides the more useful and accurate design information and has the substantial benefits of consistency and reduced statistical analysis when applied to several sites along a Dutch coastline.