Alnwick Garden Treehouse
This massive structure is located in the UK’s Alnwick Garden and was completed in 2005. Due to the house’s size, a network of wooden braces must support it, so it’s not a treehouse in the traditional sense of the word. However, it stretches high into the treetops and several trees grow right through the floor and up past the roof. The building is home to a restaurant that serves gourmet local food.