This study concerns the dynamic stiffness of foundations for large offshore wind turbines. Especially, the purpose of the analysis is to quantify the uncertainties related to the first natural frequency of a turbine supported by a surface footing on layered soil. The dynamic properties of the turbine structure are subject to relatively small uncertainties compared to the properties of the subsoil. Thus, the seabed is not a homogeneous half-space but contains layers of sand, clay, silt, etc. with depths that vary from one position to another. The depth of the top layer may even change over time due to sediment transportation. Further, the stiffness and density of the materials within a single layer is subject to uncertainties. This leads to uncertainties of the dynamic stiffness of the foundation and therefore the natural frequencies. The aim of the study is to quantify the level of uncertainties and discuss the utilization of reliability-based design of surface footings for wind turbines.