Recently, I've been able to get away from the desk and into the workshop, building some of the prototype structures for Opportunity Village, a transitional village for and by the homeless. The structures are composed of modular 4'x8' panels that can be easily assembled on site or disassembled and transported if necessary - ideal for temporary, transitional housing. The parts are also largely interchangeable, allowing for the structures to be adapted to the site. Also, if a permanent site is located for the structure, the inhabitant can simply add more panels to expand their space.
In Oregon, if a structure is under 200 sq. ft. and an average height of 10 ft., it does not require a building permit unless it is a habitable structure. While this last part is rarely enforced on private property, we are being required to have the structures for the Village permitted. But, due to the "temporary" nature of the housing, we are getting around two building code requirements that have significantly hindered the tiny house movement: permanent foundations and insulation requirements.
Habitable structures are typically required to have permanent foundations, but since the structures will not be hard-wired to utilities this is not a concern. Instead the structures will be raised on pier blocks, requiring little to no earthwork.
Second, habitable structures must typically be insulated to meet a certain R-value standard, regardless of their size. Recently, though, the Oregon REACH Code (ORC) code has incorporated size based tiers, recognizing that larger homes use more energy than smaller ones. Under this interpretation, larger homes are required to be more energy-efficient to encourage the construction of smaller homes. The ORC is an "optional building code that aims to keep the state’s design and construction industry at the forefront of high-performance building by providing both a preview of measures likely to be considered in the next mandatory building code and also a testing ground for innovative design and new technology."
The Oregon Department of Environmental Quality has an excellent presentation on why we need to encourage building smaller homes: