室 (Kanji for room suffix)

COCK because - in classic nihongo fashion - the most common jukugo (部屋) is the only jukugo to be pronounced wrong: 　へや　instead of 　ぶしつ.

And, what's the difference between 部屋　and plain old 室?

Simple! 部屋-pronounced-wrong is an average house-type room. Anything else with a 室-pronounced-correctly suffix is a specialized room.

Onyomi

SHITSU

Get your SHIT out of my room! Jesus!

You can't wear the crown until you get to the throne room.

教室(きょうしつ)	classroom ★★★★☆ 教 (teach) + 室 (room suffix) = 教室 (classroom)
待合室(まちあいしつ)	waiting room ★★☆☆☆ KUNKUN NOKURI 待 (wait) + 合 (to suit) + 室 (room suffix) = 待合室 (waiting room)
更衣室(こういしつ)	clothes-changing - room ★☆☆☆☆ 更 (all over again) + 衣 (cloth) + 室 (room suffix) = 更衣室 (clothes-changing - room)

	Meaning	Hint	Radical
屋	store	CORPSE
室	room suffix	CROWN

You go to a mortuary store to buy a CORPSE,
but you wear a CROWN in the throne room.