Is treehouse [tree house] one word or two?

I am writing a story about a wolf that looks up into a bird's dwelling. I don't even think the word[s] treehouse is/are the best thing to use. The sentence is this: "She saw a red fox's tail hanging form the opening of a bird's treehouse." I didn't want to use the word house,... show more I am writing a story about a wolf that looks up into a bird's dwelling. I don't even think the word[s] treehouse is/are the best thing to use. The sentence is this:
"She saw a red fox's tail hanging form the opening of a bird's treehouse."

I didn't want to use the word house, or home, but I couldn't think of any other word to use there. If you could help me with this word, or even give a better word to use there it would be great.

*Also if anyone knows any good sites [other than Thesaurus.com] to get synonyms, its source would be greatly appreciated*
Update: I looked in the dictionary and it said that it was on word, but then I cross referenced it and another one said it was two words...
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