"All I need/want is a single 120v circuit." Lest I sound too cynical, this question has been asked far too often - and just as often followed a few months later with "how can I get more?"
Remember, the NEC is not a design manual. There's no such thing as doing the 'code minimum;' such efforts always lead to trouble down the road.
So, what are you to do? Well, the first thing is to run your wire in a big pipe, and bury that pipe deep. You don't want to have to dig it all up again, the next time.
When the time for an upgrade comes, remember that you'll also have to pull a large ground wire, and you'll need to have a ground rod at the addition as well. Remember there's a difference between a 'branch circuit' and a 'feeder.'
Someone is sure to tell you that a certain wire is made to be directly buried in the dirt, or that you need not go down 30". Technically, they might even be correct. Yet, such a minimalist approach gives you absolutely no room for later changes. I advise you to bite the bullet now, and never have to dig again.