I have also looked into it, really not a lot of choices if you wanted to look decent and getting away with HOA. I'm thinking about building a 8x12 storage shed that matches the house, then split the inside with a wall, leave a 8x4 for toilet and vanity (no shower) and the other 8x8 storage for my lawn mower storage. Each side will have there own exterior door. Ofcourse restroom side will be tiled in nicely. here is a pic of what I have in mind but not as tall though.