Hi Heather! Make the walls and ceiling of the additional room out of CUBES, rather than walls. Walls have built in lighting. Just pull a cube from the cloud and give it the same dimensions as a wall. Then use a cube as your ceiling – just make it 1″ thick and float it above the walls.
The trick is to make the walls of the “room” matte – and you may want to make the ceiling matte black or dark grey.
And the room has to be much bigger than you think. To make it appear this dark, the room should be about 6 or 7 feet wide and 12 or 13feet deep – or it won’t be in shadow enough.
Hope this helps!
A