It is not zoned commercial. That would cost me tens-of-thousands of dollars. This is forced ADA compliance for a residential space being used in a home-based business.
Work to change the law for your residential home based business then, or fundraise. Like we disabled people have to do, to defend our rights to be in society, shop, work, and use transit while disabled -- and like so many other wheelchair users I know have to do just to get the wheels to get out of their own house. Sorry about your barn, but you seem oddly out to mess with fundamental access for other people because you need to do $3K worth of renovation, and that seems odd to me.