A jamb extender for extending extend the jamb of the window is provided. The jamb extender includes a base member, two arm members projecting orthogonally from same side of the base member, and projecting members extending perpendicularly from the end of the arms towards an opposing end of the base member. The jamb extender has a unibody construction and is preferably formed of vinyl or polyvinylchloride. The jamb extender can be used in a jamb extension assembly that includes a lineal trim holder and a trim member. The jamb extender is positioned such that a portion of the base member is adjacent to the base wall and the projecting members are adjacent to the mounting portion of the lineal trim holder to extend the jamb of the window. A decorative trim member is mounted in the trim mounting portion. A method of trimming a window opening is also provided.