Eggleston Farkas Architects The sub-stairs are a continuous monolithic pour - pretty much what you'd do for a typical concrete staircase. I don't have access right now to the construction drawings, but I believe the nosing overhangs by about 4". The ends of the treads are staggered, so the overhang varies.

Eggleston Farkas Architects The concrete sub-stair goes below the frost line (which is 16" in Seattle). Both the base and finish treads are natural gray concrete, but end up with slightly different color because of the differences in concrete batches & mixes and the finish of the formwork.

Eggleston Farkas Architects There is a concrete "sub-stair" cast below the cantilevered precast treads. You can see a corner of it lit by sunlight at the fifth tread from the bottom.