My favorite low cost, sustainable foundation method is earthbags filled with gravel on a rubble trench (double-bagged for strength). But it’s always good to have options because each project is a little different.
‘Urbancrete’ is slang for old concrete that has been torn out. Flatwork slabs such as sidewalks and driveways work best. It can be recycled and stacked like stone. Talk to some demolition companies and concrete contractors and soon you’ll have access to truckloads of urbancrete, most likely for free.
Here’s another project from Mother Earth News that provides some tips on working with urbancrete for foundations, patios and garden paths: www.motherearthnews.com/Do-It-Yourself/2006-12-01/Concrete-Brick-Paths-Patios.aspx?page=6