Here are a couple of sites to look at. The Poolhouses I will sell you plans. The Poolhouses II site 'may' sell you plans. I didn't look around too hard so I'll leave that up to you.
You will need a permit to do this even if you're doing it yourself. And if you chose a plan that's NOT familiar with your builder (should you have a builder do the work), it may cost you more because of the time and material needed in building your pool house that he's NOT familiar with. There are some places that sell pre-fab poolhouses where all you need to do is get it 'plumbered' and 'wired' and 'poured'. That's worth looking into.