Hardcore tree house enthusiasts have some guidelines for what constitutes a tree house. The tree must be an integral part of the structure, and it must have the ability to stay in the air. In some cases, minimal ground support may be necessary to ensure safety, but if the tree is removed and the structure is standing, it's considered a playhouse.
The shape of the tree will largely dictate the design, but there are many other architectural details to consider during the design process. First, the tree is still growing, and the design needs to accommodate the growth to keep the tree in good health. Using nails, screws and bolts can create puncture wounds that can harm the tree and weaken its structure, especially if they're placed close together. It's best to use a single, large bolt that fits snugly into a cleanly drilled pilot hole. As trees grow, they form new rings of bark, so eventually the bolt will grow into the tree.
Given their place in nature, many professional builders use reclaimed timber to build tree houses. But you can also employ the same kind of wood you would use to build a regular house. Pressure treated timber is well-suited for framing, and exterior plywood is good for the floor and walls. But plywood doesn't allow drainage or ventilation, so it's important to build a solid roof that's pitched at least 30 degrees to allow rain to run off easily. You can choose glass or plastic for the windows. Glass is clearer and scratch-resistant, but it's more prone to breaking. Plastic won't shatter into pieces if hit by a branch, but it's not as attractive. There are special glasses similar to windshields that are shatter resistant, but these will cost you a little more.
Designer tree houses often consist of multiple stories with a number of rooms and offer details such as winding staircases and running water. Some even boast luxury features such as hot tubs and solar lighting. These tree houses generally range from 250 to 2,500 square feet (23.22 to 232.25 square meters), and the options of what you can have are only limited by what you can afford. Prices range from $20,000 to $350,000, and can even go much higher than that [source: Conscious Choice]. The sky, quite literally, is the limit.