Correct me, if I’m wrong.
You’ve got a main shape that you extrude (Deck), then you want to put Stairs, whose projection on the ground plane partly is off of the projection of the Deck on the same plane.
So in this case you need to change the shape of the Deck, subtracting the shape of the stairs from it.
To perform that operation, you can use this approach with polybooljs: [Closed] How to split/separate a single ShapeGeometry into individual Geometries base on faces?