Questions: Why is it on someone else's property?
Does your neighbor know that it belongs to you? Did you specifically ask that this be included in your policy?
Does your insurance company know that it is on property that does not belong to you? Does your policy specifically cover or exclude this item?
Did you describe this to your agent when purchasing coverage?
I have sold homeowners policies and do not regularly see coverage for things that are not part of your home, built on another's property. Extended coverage usually refers to personal property or to separate structures on the insured property.
You must contact your agent and also an attorney for advise on this. It may be that in your state, there is coverage for this built into the standard policy, but without asking, you will not know.
Good luck with this.