FIG. 1 is a perspective view of a stackable precast concrete stairwell unit showing our new design; FIG. 2 is a top view thereof; FIG. 3 is an end view thereof, the opposite end view being the same; FIG. 4 is a side view thereof; FIG. 5 is an opposite side view thereof; and, FIG. 6 is a cross-section view taken on the line 6—6 of FIG. 2.