I started modelling the treehouse itself by creating a cube, making it thin and flat almost like a wall or fence, I then added edge loops and extruded the faces to create the other walls, floor and the walkway outside the house. I made the door and window by creating seperate cubes and shaping them how I needed them, I then placed them where the door and window are on the house and used the 'Boolean' tool to cut the shape out of the house helping to give the house a very clean and tidy mesh without having extra lines used for modelling a door and window into the treehouse geometry itself.
I made the rough shape for the tree by creating a cylinder and inserting lots of edge loops around the height of the shape, I then selected the vertices from the edge loops I made and started to manually sculpt the shape of the tree, trying to give it a rounded and exaggerated curve to it to reinforce the natural organic feel of it. I took a lot of inspiration from the following picture which is a pice of concept from the 'Open Season' art book.
It is important not to make the tree look too curved and exaggerated though as it may break too far away from the sense of reality we want to keep in the animation as well so I have to make sure that I find the right balance.