From the Center for an Urban Future.

An Accessory Dwelling Unit is a small, self-contained residential structure sharing a lot with an existing house. In Seattle, Vancouver and Santa Cruz, legislation was enacted to permit ADUs on sufficiently sized lots in one- and two-family zones. Building regulations were also relaxed to allow formerly illegal subdivisions to be safely brought to code without facing severe fines.

They argue that this could be useful in the outer boroughs of New York city. Overall, our country’s policy on housing is to raise demand (think HUD, Freddie Mac, Fannie Mae, etc.) and restrict supply (think urban zoning laws). We also do that in higher education and in health care. The result is what you would predict, given the laws of supply and demand.