Dam heightening can provide large incremental positive impacts on storage with minimum incremental negative impacts, but requires deep knowledge of the structure and its foundation. Very high gravity dams are usually well studied and documented due to their importance and complexity. Such profound knowledge of the dam-reservoir-foundation system considerably reduces the uncertainty about site conditions already at an early stage of design. Furthermore, the availability of monitoring data and safety assessment tools (FE models, predictive behaviour models) strongly reduce the preparation time to reach feasible design solutions. The Grande Dixence dam, located in the Canton of Valais, creates the largest reservoir in Switzerland, providing 10% of the country's storage energy. If heighten this dam could be used to transfer a larger share of the summer inflows to produce electricity in winter.