Martin Gumbold offers a nice paper model of a two story, European “good shed” — what I would call a warehouse, because of the loading dock. There is no apparent scale, but I downloaded it and sized it up, and think it would work very well for 20mm, and pretty well for 25mm – 28mm figures. You could use it for a variety of periods, because there are no obvious things to date it.
The Man Cave has some tips on how he built a nice looking dark ages frame house.
Mitch’s Wargaming and Modeling blog has a post on making a figure eight Pictish house. The result is wonderful.