FIFTH HOUSE?

Penthouse may not have the origin you may think. If you try and break down the word, it appears obvious that this is a combination of pent- "fifth" and house as in "habitation unit". This, however, is about as far from the truth as you can get. Penthouse actually came from the Middle English word pentice, meaning "a kind of building". Through Anglo-Norman and French, this came through the Latin word appendicium, which meant "an attachment". This in term derives from an earlier Latin word, appendere, or "to hang". Appendere is a portmanteau of ad- "toward" (though the d got dropped) and pendere, meaning "to hang". This dates back a few thousand years more to the Proto-Indo-European language, where the root spen was in use, defined as "pull or stretch". If this sounds familiar, it's also the etymon of today's word span and pendant (the latter through the aforementioned Latin root). The reason pentice changed to penthouse is that medieval people looked at pentices and realized that "this looks like a house" so maybe they should correct an obvious error and name it penthouse. This kind of transition is called folk etymology, and this particular word is also rather well detailed in Anatoly Lieberman's Etymology for Everyone: Word Origins and How We Know Them​.