In my book, there is a building that my characters refer to as "the Tree House". It is not actually a tree house, in the sense that it isn't a house built in a tree. It is a regular building with a tree growing through it. It's never "a tree house" it's "the tree house". So, my question is, since it is what they call the building, is it capitalized any time I mention it in the book? Really I'm asking if it would be just a noun or a proper noun in this context?
It is a proper noun, you have it spot on. Another example: "Quickly Robin! To the Bat Cave!" The Bat Cave isn't actually a cave of bats. It's Batman's secret hideout which is referred to as The Bat Cave. It's a proper noun the same way you have the White House, the Burj Khalifa, Buckingham Palace, Edinburgh Castle etc.
Its the name of the building therefore 'The Tree House' Its the whole name so I think it should all be capitalised.
