A wind turbine rotor blade extension fitting element, which extension fitting element is designed as a rotor blade rib with a recess and which can be pushed onto the rotor blade tip of a rotor blade to be extended, in such a way that the rotor blade tip protrudes through the recess and contacts the circumferential surface of the recess in an positive-locking manner. The extension fitting element has, on its outer circumference, an outer circumferential surface onto which a shell-like rotor blade extension can be pushed in an positive-locking manner. A core of the extension fitting element has slits and/or boreholes which, for stabilization, are filled with a material that has a higher strength and/or stiffness than the material of the core. In this way, the extension fitting element is strengthened for the transfer of high loads.