Good series of Instructables about how to build small buildings using plastic bags & earth.
Step-by-Step Earthbag Building
How to Build an Earthbag Roundhouse
PS: if you don’t know about Instructables, take the time to browse the site. There are how-to’s for everything from constructing buildings, to using software, to building weapons, to forging & casting, to just about anything under the sun that it is probably legal to do (check your local laws).