Over the past decade, a number of state and local governments have amended land use regulations to permit the accessory dwelling units (“ADUs”) on single-family lots. Measured by raw numbers of reforms, the campaign to secure legal reforms permitting ADUs appears to be a tremendous success. The question remains, however, whether these reforms overcome the well-documented land-use parochialism that has, for decades, represented a primary obstacle to increasing the supply of affordable housing. In order to understand more about their actual effects, this Article examines ADU reforms in a context which ought to predict a minimal level of local parochialism. In 2002, California enacted state-wide legislation mandating that local governments either amend their zoning laws to permit ADUs in single-family zones or accept the imposition of a state-dictated regulatory regime. We carefully examined the zoning law of all California cities with populations over 50,000 people (150 total cities) to determine how local governments actually implemented ADU reforms “on the ground” after the state legislation was enacted. Our analysis suggests that the seeming success story masks hidden local regulatory barriers. Local governments have responded to local political pressures by delaying the enactment of ADU legislation (and, in a few cases, simply refusing to do so despite the state mandate), imposing burdensome procedural requirements that are contrary to the spirit, if not the letter, of the state-law requirement that ADUs be permitted “as of right,” requiring multiple off-street parking spaces, and imposing substantive and procedural design requirements. Taken together, these details likely dramatically suppress the value of ADUs as a means of increasing affordable housing.