Last week I realized that the builder of this cottage would probably have used the same kind of geometry for the elevations that he used for the floor plan.
This is an obvious observation. Really! Makes me roll my eyes to know that it only took me 3 years to figure this out!
It is clear from the photograph that the house has been expanded over the years. The siding was 'updated' about 40 years ago. My diagrams refer only to the part of the house under the gable, to the right.
So here are the diagrams and drawings - click to enlarge:
The side elevation measure by me and drawn to scale:
The circle geometry I think the builder used for the elevations:
The upper diagram shows 2 circles intersecting - the outside of each touches the center of the other. The place where the circles overlap is called a vesica piscis ('fish bladder' in Latin). The red line bisects the vesica piscis.
The lower diagram shows one circle with 4 circles intersecting it, creating 4 intersecting vesicae piscis. The red lines are only partially shown for clarity.
The diagrams show how the shape and dimensions of the elevation were determined.
I have drawn only part of one of the circles on the sides, the left one. The vesica piscis there follows the wall of the house.
Note that the center of the main circle not only positions the 2nd floor windows but also marks the square of the wall, the top of which is where the builder will set his rafters.The square is outlined in green. The circle which determined the floor plan, its dimensions, is also used to determine dimensions of the elevations.