Minimum Area Standards—How tiny is legal?
these are the primary standards that I've found in the IRC that limit minimum area:
R304.1 Every dwelling unit shall have at least one habitable room that shall have not less than 120 square feet of gross floor area (note: this requirement has been removed in the 2015 IRC)
R304.2 Other habitable rooms shall have a floor area of not less than 70 square feet (except kitchens)
R304.3 Habitable rooms shall not be less than 7 feet in any horizontal dimension (except kitchens)
R304.4 Portions of a room with a sloping ceiling measuring less than 5 feet between floor and ceiling shall not be considered as contributing to the minimum required habitable area for that room.
R306.1 Requires that every dwelling have a water closet, lavatory, and bathtub or shower (which could be as small as 18 sf while still meeting spacing requirements in Section 307)
R306.2 Requires that every dwelling have a kitchen area with a sink
There's no requirements that the sleeping area or kitchen must be in a separate room, so by my interpretation of 2015 standards, you could have a legal dwelling as small as 88 square feet (70 sq. ft. habitable room + 18 sq. ft. bathroom).
Some municipalities may have additional standards that go above and beyond this, which begs the question: why would local codes have a minimum area standards that go above and beyond the International Residential Code?
Size Based Tiers
There is a presentation by Oregon DEQ ("Small Homes: Benefits, Trends and Policies") that introduces the idea of size based tiers for insulation and ventilation standards. It was adopted into the Oregon REACH Code (direct link) with the basic premise being that large homes use far more energy to heat than small homes, and therefore these standards should be scaled appropriately. They only go down to 1,500 square feet or under here but I think this could be pushed further for tiny houses.
A Model Tiny House Code
While there is the possibility to work within the existing code now, code changes can be made to allow for more efficient tiny house designs in the future. Below is a draft of a Tiny House section to be added to the Oregon state building code.
Rather than revising the existing code, this is a proposal for an additional section that contains exceptions and modifications for houses 350 sq. ft. and under.