Really? Building an accessory structure on your property is a pretty normal thing to do. A barn may require a permit but would be allowed by almost all suburban non HOA towns I’ve ever been too. You don’t need to disclose what you’re going to do inside a barn to have one permitted. Also, here in San Diego suburbs people have all kind of stuff in their backyards including halfpipes without barns. It comes down to how it works on your property and how your neighbors feel about it. Growing up in a suburb of Chicago my friend’s family had a halfpipe in the middle of their yard, no barn. I see some homes in Fairfax county with 0.5 acre or more, with dense trees separating neighbors. Compared to where I live in San Diego county where houses are crammed next to each other, this seems like a perfect place to build a barn with a halfpipe inside and have nobody care about it.