Unlike miniature architecture structures, which are models of a finished structure from all sides and angles, room boxes are often the view of a room, looking in through one wall. This is no exception, but what I like to do is feature different angles and hidden lines of sight, rather than the usual "three walls". In this case, there is a view from the front of the back fence and implied vegetation beyond, but there is also a view of the garden angling around the side of the house, which opens up the small structure and makes the whole scene seem larger than it really is.
This is the finished product. Now let's see if I can remember how I built that...
First, creating the box. In this case, because of the small scale, I'm using thin wood. Not the relatively small size of the box.
Using weighted verticals to establish interior dimensions.
Estimating the sight lines based on future location of rear fence and walls against a backdrop.
Filling in the back vegetation.
Building the fence.
Fence in place. Looks pretty good with the garden in front and vegetation scene in back.
Starting to project finished dimensions.
Infrastructure and wiring
Shoji. Do them right, no easy way about it!
View from the top, with supports/wiring in place.
Done! Kirin time.