Here’s a site with four very nice paper models of 25mm / 28mm half-timbered houses that would be appropriate for a fantasy or medieval town setting. There are “shoddy shops” a “posh place,” a plain half timbered house and a building that is more fantasy oriented. All are in pdf format.
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.