Why wouldn't you tape out the stair case like it is in drawn on the ground plan?
What I am asking is, how much smaller is the rehearsal room then the space?
When shrinking the set to fit for the rehearsal room, I always recommend the following trick. Get a piece of plexiglass that is big enough to draw the rehearsal room in the same scale your ground plan is drawn is in (1/2 or 1/4 inch). Draw the walls, doors, and other architectural elements of the room on the plexiglass, which will allow you over lay this on top of of the ground plan and figure out the best way to lay out the room (which orientation, etc.) Once your theater have a 1/4 and 1/2 scale version of these overlays, you can just keep with rehearsal locations.
Anyway, take the overlay and discuss with the director. He or she will have a the best idea on how they are going to use the space and where you can loose space and where you shrink areas. And since the tape is just as much for the directed as the actor (and let's be honest, Actors often ignore the tape), I think the Director should get a vote in how we tape out.