I have been looking at sheds I can convert into a new office space in the backyard and then I stumbled on this photo of a vintage short bus converted into a guest bedroom. I can see the cost of doing something like this to be similar in cost to a new shed and would be a whole heck of a lot cooler LOL.
It would take some time to find the right vehicle I bet but this could be such a cool addition to the home. Another idea could be to find a vintage airstream trailer to do something similar.
Here’s the link to where I found this photo which also has some other fun pictures of really small cabins/structures.