Is your house located in a subdivision? The permits could be for the subdivision. Who did you buy your house from? Did someone own a plot of land, and then constructed the house? I would determine who originally built the house and start there. It is unclear from your posting what exactly you want to do here. Do you want to move your septic tank? If so, you would need to comply with local, and state land use and environmental laws, and I would contact a contractor experienced in these matters. Good luck to you!
You will want to review the chain of title for your property. Determine who built the Agricultural building and pulled the permits for it. Then you will want to determine if the owner who sold the home to you was aware of this. Finally, you will want to speak with your realtor to find out what they knew about this and what the sellers realtor new or should have known about this. In many areas, it is required by the lender to have the septic tank pumped and tested prior to purchase, you could look into that as well.
Note: These suggestions are being given without full knowledge of the facts of the situation, and are not intended to be relied upon or to create any attorney client relationship.