How do I copy a room?

You need to save it to your library. You will also be able to see a copy of it in the rooms and wall section of build mode if you sort the catalogue for saved rooms.

If you mean to use on a different lot then, as far as I know you need to at least save it to your library. But if others know different I would be very happy to be proven wrong :)

you can save a house or just a room. just save it to your library. then you can just go to your library and recreate as many copies as you want

Well, I mean on the same lot without saving it to the gallery first. Just grabbing a room and duplicating it in build mode. I thought it would be as easy as moving a room … shift/alt/ctrl click while moving doesnt seem to do a thing though (in graphic programs it's usually pressing alt while moving something). I could swear I saw them doing this in one of the build mode demos/let's plays. (I could be wrong though)

I am basically looking for an easy way to add rooms in the same style without having to paint the walls, tile the floor etc every time.

Eh, the only way I know of is to save it to the Gallery. If you don't want to share it, you can just save it locally to your own PC and not upload it. Otherwise, if you want to copy wall and floor textures, I just use the eyedropper tool and shift+click to fill in the room.

When u go to build mode just press the wall of the room n it will show you the two squares on top of each other icon, thats a copy for the room.