Check the below for the Oregon Treehouse Resort plus leads for professional builders.
If you have plans to build a treehouse, you may also have important decisions about the heath of the species you plan to build in - maple, pine, beech, alder, fir, etc... You can find links at the bottom, where other people have much more information about constructing Treehouses. This page is mainly a reminder to keep in mind the long-term health of the trunk, and branches. And to consider calling a professional for a short one hour visit: maybe more if you choose. If bark and tissue growth is important to you, that does not mean tyou can't have a treehouse. But why invest or build a top-notch treehouse that should last for 50 years or more, but loose it in 10 years because you did not allow for growth.
The health or development of the trunk and limbs will depend on how you secure or position the treehouse within the canopy. The trunks, leaders and limbs expand yearly, and there can be significant tissue damage when treehouses are attached without allowing room for the trunk and bark to expand outward.
For entertainment, I included the image above of a round treehouse. The photograph was taken at a place called It's a Burl or Burl Gallery in Kerby, Oregon, a few minutes east of Cave Junction. It looks like the trim for the window openings is from madrone or oak - from hollows in the trunk where limbs died and the wood tissue. One other other large treehouse at a property I used to work at is this photo album.
If you want to protect the the main stems, plan your project more carefully. It is available to position treehouses supporting weight on posts. An entire treehouse can be supported on posts, rather than anchoring lumber against bark. Like using stilt supports.
If this project will be a significant investment, consider calling a professional treehouse maker and horticulturist. You can gain a few pointers about detecting hard to spot internal decay and weakness that are not obvious, ideas for fasterners and whether you should put a cable through the trunk to anchor or suspend the treehouse.
A professional, for liability reasons, may not be able to tell you that you should fasten a treehouse with certain hardware, but should be able to provide a personal opinion about how they would approach the project if it was their own yard and treehouse project.
Oregon has a nice assortment of treehouses. There is even a treehouse resort near Cave Junction in Southern Oregon, west of Grants Pass. Take time to browse their treehouse pages in depth. There are photos, rates, news and links to video, Here is the link to Out'n'About Treesort @ www.treehouses.com.
I have heard from several people who really enjoyed their stay at that treehouse resort: including families.
Also take a look at: Building Treehouses, Cedar Creek Treehouse at Mt. Rainier, Wikipedia Treehouse article, and treehouses.org.
March, 2010, I attended a presentation by researchers Prof. Sillett and Dr. Van Pelt, about the tallest forest species, at University of Oregon. At the reception for the event, I met a few folks from Seattle, who specialize in treehouses, Their conversation and website were rather interesting: Treehouse Builders.
There are plenty of pages about treehouses on the internet - more than I expected. Many of the pages have tips, advice and photos. Its a hobby that many people seem to enjoy sharing about
Here is one more: TreeHouses by TreeTop Builders: Building unique and outstanding treehouses. Safety, quality, experience and professionalism.