The longhouse had been popular for a long time by the viking age, when the "three-shiped" longhouse had become normal (two internal pillars), but was changing a bit by then. While secondary houses and separate barns became popular the classical "half hall half barn" such as in Borg became less common, and the hall became the main focus. Earlier the house was a multipurpous building designed to house a family. It could be as little as 10 meters, and has existed since the bronze age. The devlopment of the hall is what most people think about when it comes to longhouses.

While the animals were moved back out of the house, and they therefore shrunk in size and became shorter and more rounded. Many also point to the centralisation of power as an important part, since local chiefs had less need for them. The church might also have taken over the role as a gathering place.

As you get into the middle ages (starts 1066 in Norway), the small "lafted" log houses had become the most common by far (in Norway, Iceland and Denmark would have different styles). Small "one-shiped" (no internal pillars) longhouses continued to exist until about 1200.

But what's the difference?

• Long houses are a roof held up by pillars, the walls then filled in with planks.
• The more modern log houses are logs stacked on top of each other, the wall holding the roof up.

Source: Dug up some lecture slides from class last year.