A blade extension for a rotor blade and a rotor blade assembly for a wind turbine are disclosed. The rotor blade assembly includes a rotor blade having exterior surfaces defining a pressure side, a suction side, a leading edge, and a trailing edge each extending in a generally span-wise direction between a tip and a root. The rotor blade assembly further includes a blade extension including a first panel and an opposed second panel. Each of the first panel and the second panel includes an interior surface and an exterior surface each extending between a proximal end and a distal end. The distal end of each of the first panel and the second panel is spaced apart from the rotor blade in a generally chord-wise direction in a standard operation position.