屋里(wū lǐ) - indoors, inside the house, in the room in Mandarin

屋里 (wū lǐ) is the inside of a house or a room, and it is the natural opposite of 外面. It is a place word, so it goes where places go: 在屋里 in the room, 进屋里 into the room, 屋里的灯 the light in the room.

It has a homely, spoken feel to it. 房间 is the neutral word for a room in a flat or a hotel, and 室内 is the formal indoors of a sign or a rule. 屋里 is what you say at home, and in northern speech 屋里人 can even mean the wife.

The trap is bolting 在 or 里 on twice. English "inside the room" tempts learners into 在屋里里面 or 在屋子里里. 屋里 already contains its locative, so 在屋里 is complete. If you want the fuller noun, say 在屋子里 with 屋子, but do not mix the two.