A half bath would be very helpful, and if you intend to have clients in and out, it's pretty much a necessity. Even though it's not a big deal for you to go into the house to use the bathroom, do you really want to make a client do that? Once you have plumbing, you have something closer to a barebones apartment than to a shack. Unless you're just telecommuting, it's not really reasonable to build without one.
Perhaps think backward. Take a studio apartment concept, and figure out what you don't need. You don't need a kitchen, but the bathroom has a sink, so you're still good to go with convenience items and stored prepared food. Coffeemaker, mini-fridge, microwave. You don't need a bed, or if you opt for one it need not be a full-time bed. A futon might suffice. (There may be times you need to lay down, but going back in the house to do so would break your flow somehow. Like supervising compiling or rendering or 3D printing or something.) Then everything you would want in any office -- your choice of furniture and equipment.
What's also important is what doesn't go in there. Network gear is probably better left in the house, but there are cases where you might want to move it. But more importantly, don't take anything irrelevant out there. I don't mean you can't have a Rubik's Cube on your desk, I mean don't put anything out there that is totally unrelated, except in dire emergency. Otherwise you will soon feel like you are working in a closet -- because you essentially are.
Look at this space as more valuable than the house it lies behind, on a per-square-foot basis -- why would you want to store junk in the high-rent district?