Designing and building a tree house has unique challenges compared to building a structure that sits on the ground. Because you're building in and on a living, breathing thing, it's a process that requires extra thought and care, using sound environmental principles. The tree must have the freedom to continue to grow, not to mention the house will be subject to elements like wind and rain that are more concentrated in higher skies.
The first and most important step is to choose the right tree. Almost any mature deciduous or coniferous tree is a good choice, as long as it's healthy. Deciduous trees (trees that lose their leaves in the winter) generally have dense wood that can support larger loads. It's essential that the tree is healthy and will continue to thrive despite the addition of extra weight. It also needs to have a solid and intact root structure. Signs of a tree with health issues include multiple dead branches, leaf discoloration or liquid oozing from the bark. Working with a professional tree specialist, an arborist, can help make these determinations.
A healthy tree will treat a properly installed tree house the same as it would additional branches, which means growing additional wood to support it. For this reason, multi-storied houses are best built over the period of a couple of years, to allow the tree to adapt to the load.
Wind is also a big consideration in tree house design. A tree house acts like a sail on a boat in strong winds, creating resistance that adds a considerable load to the tree's roots. A tree will compensate for the weight of the tree house over the first few years by adding thickness to its root structure. So when coming up with a design, keep in mind that a tree house that's lower in the tree will produce lower forces on its roots and stands less chance of being knocked over by gale force winds. Wind also means that trees sway back and forth, so the support structure of the tree house needs to allow for this to happen.