Well, I had the idea that by joining additional mid sections, you can create a house as long as you like, whilst it will always be 15 inches wide.
You can also place it against a wall without losing any design detailing. The back of the model is a plain wall. The 'front' can be fitted with 'doors' with windows as can be seen on the real end-of terrace shop houses.
The shortest model could be just the front, a starting point from which the model could extended. The interior can be in the modelers gift, including walls, layout etc.
The card model is a proofing for the design. Detail drawings already done for the front section. But the proofing build will have to wait for our return from our NZ trip. Hopefully in April.