Legal Question in Real Estate Law in California

Sure, it is a trespass and since it is a permanent structure it might be considered conversion. You could get a court order for the removal of the staircase and damages for it being placed there. If you are absolutely sure that it is one your property and that the person who erected it had no valid reason to do so, then you could block off the part on your property and erect a no trespassing sign.

Conversion only involves personal property, not real property. What you have described is an encroachment. I suggest talking to a competent real estate attorney near you to discuss your rights and remedies.