Abstract. Advanced aeroelastically optimized tip extensions are among rotor innovation concepts which could contribute to higher performance and lower cost of wind turbines. A novel design optimization framework for wind turbine blade tip extensions, based on surrogate aeroelastic modeling is presented. An academic wind turbine is modelled in an aeroelastic code equipped with a near wake aerodynamic module, and tip extensions with complex shapes are parametrized using 11 design variables. The design space is explored via full aeroelastic simulations in extreme turbulence and a surrogate model is fitted to the data. Direct optimization is performed based on the surrogate model, seeking to maximize the power of the retrofitted turbine within the ultimate load constraints. The presented optimized design achieves a load neutral gain of up to 6 % in annual energy production. Its performance is further evaluated in detail by means of the near wake model used for the generation of the surrogate model, and compared with a higher fidelity aerodynamic module comprising a hybrid filament-particle-mesh vortex method with a lifting-line implementation. A good agreement between the solvers is obtained at low turbulence levels, while differences in predicted power and flap-wise blade root bending moment grow with increasing turbulence intensity.