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?