Slughorn's office is a large room, bigger than the usual teacher's study.

From the Story

Harry Potter and the Half-Blood Prince

Discovered in chapter 15, The Unbreakable Vow

For his party, Slughorn's office is draped with emerald, crimson and gold hangings, and looks like the interior of a vast tent. The room is bathed in red light, cast by an ornate golden lamp which dangles from the centre of the ceiling, with real fairies fluttering inside.