Abstract: A window sill assembly for protecting and covering an existing window sill at the base of a window. Two elongated members, each provided with a slideway, are attached to the window sill in a parallel and spaced apart relationship, one member on either side of the sill and extending away therefrom. A rectangular plate is slid, in a manner similar to a drawer, along the slideways until the rear portion of the cover encounters the wall below the window. The cover is then fastened to the sill, for example by being screwed thereto. Alternately, a flat rigid strip is rigidly fastened to the sill under the cover and is provided with a prong extending therefrom. A lip extending from the cover engages the prong and interlocks the plate with the strip.