To make one, please refer to modelling software like Art of Illusion, Blender or Sketchup. You may be able to find a model online (3D Warehouse?)- but the number of steps puzzle me. Is it a very high staircase (level above)? - I count more than 4 metres from the number of steps... That and the shape may be hard to find. If you would like someone to have a go at making it, you need to specify dimensions roughly and direction.

If you have the staircase model and just need to insert it - that is dealing with the shape - see Guide for SVG Path cut-outs and other furniture properties that deals with staircases and cut-out shapes.

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