On Tent City Urbanism, Andrew Heben shares the prototype design of the Emerald Village tiny house and describes how it will meet current building codes. An excerpt is below.

“I find no requirement that the sleeping area or kitchen must be in separate rooms, and instead could be combined in a studio arrangement. So I believe it could be legitimately argued that a minimum legal area by 2015 IRC standards could be as small as one habitable room of 70 sq. ft. with sleeping area and kitchen, plus a bathroom as small as 18 sq. ft. while meeting minimum spacing requirements.”

Relevant sizes from IRC 2015:

R304.2 Other rooms.
Other habitable rooms shall have a floor area of not less than 70 square feet (6.5 m2). Exception: Kitchens.

R304.3 Minimum dimensions.
Habitable rooms shall not be less than 7 feet (2134 mm) in any horizontal dimension. Exception: Kitchens.

R304.4 Height effect on room area.
Portions of a room with a sloping ceiling measuring less than 5 feet (1524 mm) or a furred ceiling measuring less than 7 feet (2134 mm) from the finished floor to the finished ceiling shall not be considered as contributing to the minimum required habitable area for that room.

R306.1 Toilet facilities.
Every dwelling unit shall be provided with a water closet, lavatory, and a bathtub or shower.

R307.1 Space required.
Bathroom fixtures shall be spaced in accordance with Figure R307.1
– 30″x30″ minimum shower size
– 21″ clearance in front of toilet and sink (can overlap)
– 24″ clearance for shower door
– 15″ from center of toilet to side wall

R306.2 Kitchen.
Each dwelling unit shall be provided with a kitchen area and every kitchen area shall be provided with a sink.

See http://www.tentcityurbanism.com/2015/02/emerald-village-tiny-house-prototype.html for pictures and the full text.