A few months back I stumbled upon the Dave Graffam line of paper models (http://www.davesgames.net/). I have been involved in mini wargaming (25/28mm) for less than a year and WW2 for only a few months. Having an interest in ACW as well, I don't have deep enough pockets to purchase a bunch of nice resin buildings for each era. I have purchased a few JR Miniatures and Miniature Building Authority pieces and will say they are very nice, but too pricey for me to acquire a bunch of pieces. I have found the Graffam models excellent alternatives and very affordable as well.
Pros:
Economical
Flexible in that many of the kits allow you to tinker with the appearance by switching layers in the PDF files
If you make a mistake, just print another sheet
Cons:
More delicate than resin
Some kits can be rather time consuming to construct, especially if you take some of the extra steps I have (the times involved shorten as you get used to working with them)
Here are some pics of the Ruined Church kit. I chose to go with a red brick look so I could use with either WW2 or ACW. This can be done in stone as well.
I chose to make the shell hole more realistic with wood filler and acrylic paint. Rather than using cardstock as the directions suggest, I chose 1/4 FB for walls and floors and a thinner (around 1/32") project board for roofs and staircases. Have fun!