An exterior sill assembly preferably for a window has a preferably extruded body, and attachable facial and preferably end caps for covering open ends of a cavity defined by the body. The body has a substantially vertical flange for mounting to a structure preferably below a window frame. A sill projects outward from the flange to a distal portion and preferably a support plate spaced below the sill and projecting outward from the flange to a distal segment. Attachment features are carried between the facial, the distal portion and the distal segment for releasable engagement of the facial to the body. Fasteners disposed in-part in the cavity extend through the flange for engagement into the structure. Preferably, the fasteners are used in conjunction with respective members for distributing stress loads across the flange.