I bought two of Amera's Plastic buildings (http://www.amera.co.uk/). They come as vacuum formed plastic, but I had bigger plans.
Since the building come 'hollow', I filled them in with recycled tissue paper (my wife goes though a box per week) and then covered the paper with Timbermate.
I spray painted them brown
And added some foliage for that "nature gets its revenge" look.
For the larger office building, I added some broken grass from scored packaging from some food
The office furniture is from Eureka in Melbourne.