Abstract To assess of the lifetime extension (LTE) of wind turbines, the remaining fatigue budget of different turbine components is predicted. The state-of-the-art approach to predicting the LTE uses a comparison of damage-equivalent loads (DEL) under site conditions and design conditions. The DEL-based approach entails several simplifications, i.e., neglecting the load direction and the mean load. An analysis based on stresses can mitigate the above simplifications. In this work, the LTE of a 1.5 MW turbine is assessed on the basis of the DEL-based approach and compared to that of the stress-based approach. The assessment focuses on components which are critical for the lifetime, i.e., the blade bolts, the blade root laminate, and the main shaft. Generic models of these components are implemented to calculate stress histories for the stress-based assessment. In addition, the main assumptions that may have to be based on estimates are outlined. The analysis shows that, depending on the component, the results of the stress-based approach may differ notably from those of the DEL-based approach. The stress-based approach can be used to improve model fidelity in LTE assessments.