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