In December 2010, Wargames Illustrated did an article on how to build 15mm desert buildings. Unfortunately that article hasn't made it onto their website, but they were dead easy. Foamcard for the basic box, some blue foam for bits and pieces, and scrap card for detailing. Oh, and plenty of white glue. I ran masking tape over some of the joins to hide the raw ends, but it's probably not necessary under a coat or two of textured paint. I'm going to make another four of these, then try something similar with corrugated card roofs and walls.
They'll end up being used in 15mm sci-fi games; I have a game of Hammers Slammers planned for the new year so I need to get some scenery together before then.