How to build the ultimate treehouse (Outside Mag) Nelson Treehouse and Supply sells plans for nearly any tree configuration (from $100). Single-tree designs like this one are the most popular. All you need is a straight trunk at least 24 inches in diameter. A treehouse attachment bolt (TAB)—a twelve- or twenty-four-inch steel rod that’s screwed into a tree’s heartwood. Each TAB runs about $250 and can support upwards of four tons. More importantly, it allows the tree ample space to grow.