ADU's Changing the Rules?
ADUs are impacted by two types of code: Construction and zoning. Some areas hesitate to permit these units due to neighborhood resistance and/or the fact that many ADUs are constructed by landowners, instead of professional builders.
Some forward-thinking markets are simplifying the development requirements to make ADUs a viable option for homeowners. In areas where the building and zoning codes aren't ADU-friendly, homeowners can apply for a variance (approval to building outside the standard regulations) or an exemption from their local regulations.
Unfortunately, there is no single source to verify ADU building codes and zoning restrictions for a given area. To learn what is and isn't allowed in your market, it's best to work your way up -- start with local ordinances, then check for county and state regulations.
Since most local building codes in the U.S. are based on the International Residential Code, some "tiny house" enthusiasts are currently pushing to have a tiny house code added to the IRC. If successful, this would make ADUs easier to build legally in all areas of the U.S.
(Excerpt from The SRES Professional)