The development of a model-based approach for a damage severity assessment applied on a complex composite skin structure with stiffeners is presented in this article. Earlier nvestigations on composite structures with stiffeners revealed that a vibration based structural health monitoring approach employing the Modal Strain Energy Damage Index algorithm can detect and localise delaminations. The next step performed in the part of the research presented is to assess the severity of the damage. It is shown that parametric studies enhance the reliability of the damage severity assessment based on measured data, since a wide range of damage cases can be studied in advance, possibly under variable environmental conditions. The benefit of numerical models is also found in the ability to select measuring points in a smart way, hence optimising the efficiency of the data acquisition system. The amount of damage related information is maximised for a minimal number of sensor positions.