For the first time in it's ten year history, the annual Treehouse Conference will be held in Fall City, WA (25 miles east of Seattle), at Treehouse Point on October 10th, an events facility owned by Pete and Judy Nelson, the folks who own Tree House Workshop. (Peter Nelson is the author of a series of great Tree House books.)
Treehouse Workshop offers full design and build services for your treehouse project, but they also help you if you want to design and build it yourself.
Their workshops will equip a homeowner with the skills needed to design and build your own treehouse, and they cover how to select a tree and evaluate its health, basic tree-climbing, tool safety, design, construction of your platform, and how to build and finish your structure.
Tree House Workshop is also a licensed and bonded contractor in Washington State, so if you don't want to build your own, they can build it for you. Peter's book is a fabulous peek at treehouses he's designed and others have built around the world.