It is a thought that, if you were to upgrade the house, add all the rooms, etc, will it look the same as someone who ALSO bought all rooms for their house? Or can you actually position your rooms in a unique way to your liking? I just hope that all houses will be unique, not just the game telling you where the room goes.
This is just my guess from the trailer. The house is built one room at a time. In the trailer the DB is scrolling through a book showing multiple rooms all for the west wing, bedrooms, enchanters tower, greenhouse. From that I think you do not pick a room and decide where it goes, you pick a location and decide what goes there.
In one of the images of a completed house it appears the Enchanters tower is on the back end of the house rather then the side like shown earlier in the planning stages. From that I'm guessing we have options for where the rooms go.