Finance barn expansion through rental?

Unfortunately, you cannot bear any risk (rental loss, maintenance, etc). Your first step should be to go to the bank. Communicate your plan and have them tell you how much credit you will get. In my crystal ball, I can see that you have far too little equity for this project. The budget of an investor looks different to me. Regards Yvonne

Nice that you see it so positively that the bank is concluding the loan with you. I say there will be a lot of need for discussion.