DOES ANYBODY KNOW???

This is idle speculation, but...

Doors are basically great gaping HOLES in the side of a building. In a hearth-warmed cottage it'd make sense to make them as small as feasible, to reduce heat loss. Most people can stoop for a few seconds to go through a shortish doorway.

Yeah, but the ceilings are extra-low as well. Anyone who's cracked their skull negotiating the bar of an old pub will confirm this.

In my youth I was told it was because the maximum height of a wooden framed building (and thus the maximum distance between floors) was determined by the maximum length of timbers that could consistently be obtained from mature oak trees. Anything higher required expensive joinery or stone construction and was thus limited to the rich/royalty/church.

This, however, could be total bullshit.

http://209.164.8.65/index.shtm