Bought house 2007 which has barns & out buildings. Using barns last yr did not submit it on 2017 taxes. How do I add it as storage/livestock shelter in 2018 4,800 sq.ft.

I have a Farm business. I have goats, Emus, chickens and also a large garden. I have a Farm Market in the summer. Sell Eggs part of the year. The barn is for animal shelter & storage and I already have an office expense in the house itself for, business accounting. Plus I use the space to grow plants to sell & grow fodder for my animals. How do I add a second expense for the barn? I tried adding it as an expense but it asks for the cost of the barn. It was included in the coast of my property when we bough the house in 2007. I have an outside greenhouse which I have to repair again before using and am in the process of building a second greenhouse on our deck.