I just figured, since I've got the space, and I basically work from home and I'm around most of the time. I don't need staff to keep an eye on the place or drive somewhere far. It doesn't have to be storage specifically. I was also thinking workshop, or something like a yoga studio space, that kind of thing.

Better to check how that lines up with the laws in your state. Rules are different everywhere. But I've got a feeling that if you're making money off it, you'll probably still have to report something. Might be worth registering the space as commercial property.

That's technically commercial use of a private structure, and stuff like that is better to double check with your local government office. Sometimes it's enough to just declare the income, and sometimes you'll have to change the property status to commercial. I love running the numbers, so if I were you I'd first figure out the economics, like what the build would actually cost. As an example, you could look at modern metal structures like https://www.uspatriotsteel.com/blog/how-much-do-metal-buildings-cost/. Once you've got real numbers to work off of, it'll be a lot easier to see if the whole renting it out idea would even pay off, and whether it's worth starting in the first place.