Types of Treehouses
The first things to consider when building a treehouse are the scope and function. A lot depends on the tree and how much floor area you can realistically support. The most basic type is a floor only, which is appropriate if you only want a children's playhouse. Depending on the height, you will need to consider a railing and whether a ladder or stairs with a railing are necessary. If you are ambitious, you might want to consider building walls and a roof, which will dramatically increase the work involved, then adding windows and a door if you want to create a guest house. All of these steps involve exponentially more money and time.
The Floor
The exact size and dimensions of the platform will depend on the tree that you are building the house in. While typical treehouse designs tend to use supports at 45 degrees from the tree trunk, I used an unconventional design: I cut off several trunks from a willow tree and used the stumps as the support for my platform. I used the cut offtrunks as pillars to support the extended balcony. If you are building a floor without a roof, then you should use decking and leave spaces between the boards for water to drain. For the indoor flooring I used tongue and groove boards. If the treehouse is higher than 2 metres, you will need a railing. An elegant solution is to use a wooden frame and rope, as I did for my balcony. Although it can work out kind of expensive - I needed 100 metres of rope for this railing.
The Stairs
Adding a stairwell will add some work, but will make it much easier to use the treehouse in the long run. If you have to climb up and down each time you want to use it, it will make you less likely to want to use it. If you are planning to build a roof, this should be done first (after the floor), so that you can get up and down easily when building the roof.
The Walls and Roof
If you want to use your treehouse as a place for guests to sleep for example, then you will need a roof, windows and doors. Windows will add a lot of work, but are worthwhile, since the advantage of being in a treehouse is the view. You can make part of the treehouse a balcony, which means you will need to cover the whole floor area. I made my windows entirely from scratch, which allowed me to fit them exactly to the size of the treehouse and is also more asthetically pleasing. I made the balcony doors out of glass. The entrance doors were a lot simpler and made out of lime wood. You can use roofing felt as a cheap and easy way to waterproof the roof, but I went for shingles, which are more expensive and more work to install, but much nicer looking. I am currently still working on the interior, but plan to install a bed. I have started insulating the walls (although I am not planning to use it in winter) as it is not much extra work and I want to cover the wiring for the electricity (which is solar powered).
