From Strong Towns:

The R Street Institute (a public policy research organization) recently released a policy study about accessory dwelling units which is worth a read, especially if you’re looking to convince local leaders that they should be permitted and prioritized in your town. Accessory Dwelling Units or ADUs are small, usually free-standing cottages that homeowners build in their backyards or sometimes above a garage. They can provide housing for renters or family members and are gaining in popularity across the United States. However, there are many challenges and limitations to ADU construction and habitation in most cities.