Penthouse

Pronounced with the '-t-' and the '-h-' separately (IPA: /'pɛnt ,haʊs/), not with a '-th-' (/θ/) sound.

This is a word with an interesting history. It originally meant a lean-to shed - one added to an existing building, leaning on it, usually with a sloping roof. In other words, it was not a desirable living space. It came to be used for extra rooms, or stores, built at the top of a building. As buildings have become taller and the view from on high is much sought after, a penthouse flat is now a supremely desirable dwelling, which fetches high prices. (The development of lifts has probably assisted this change.)

Etymological note: The word 'penthouse' comes originally from the Late Latin appendicium, 'appendage' (a noun from the verb appendere, 'to hang from').