playhouse vs tree house

playhouse and tree house both are nouns.

Word	Noun	Adjective	Verb	Adverb
playhouse	Yes	No	No	No
tree house	Yes	No	No	No

As nouns, tree house is a hyponym of playhouse; that is, tree house is a word with a more specific, narrower meaning than playhouse:

playhouse: plaything consisting of a small model of a house that children can play inside of
tree house: a playhouse built in the branches of a tree

playhouse (noun)	tree house (noun)
plaything consisting of a small model of a house that children can play inside of	a playhouse built in the branches of a tree