State Laws Legalizing Accessory Dwelling Units

Over the past decade, state policymakers across the country have established a new, accelerating trend of state laws that set limits on local zoning authority. These laws include new requirements that localities allow more, less expensive types of housing to be built. The most common state laws in this vein require localities to allow homeowners to add accessory dwelling units (ADUs) to their properties.

So far, 18 states have broadly legalized ADUs for their homeowners. In a new policy brief, Kol Peterson and I analyze the relative strength of these laws. This is important because in zoning, details matter, and this is particularly true for small, infill projects like ADUs. It may be that a web of laws are blocking ADU construction, and getting rid of just a few of these barriers may not be sufficient to lead to a big increase in supply.

In his book Backdoor Revolution, Kol identifies three laws that commonly prevent homeowners from building ADUs, even where they’re nominally allowed:

• owner occupancy requirements;
• parking requirements; and,
• requiring a special or conditional use permit for ADU construction.

Across the 18 states that have adopted some type of preemptive ADU law, we categorize each according to these three policy variables and according to whether or not they require localities to allow both attached and detached ADUs. We label the 10 state laws that address these four potential barriers to ADU construction as having strong ADU laws:

But because there are so many potential blockades to ADU construction, even homeowners who live in a state with a strong ADU law may still find themselves unable to build one in a way that works for them. For example, many local ordinances that nominally allow backyard cottages have large setback requirements that make them infeasible to build. Localities may charge tens of thousands of dollars in impact fees for new residential units that may be an insurmountable burden for people who would add a basement apartment if the cost of doing so was lower.

Given that all of these regulations matter, Kol and I compiled data on 14 additional variables among ADU laws for those 10 states that meet the “strong law” threshold. All told, we find that Arizona, California, and Washington have some of the combinations of rules that make it easiest for their homeowners to add ADUs. The complete breakdown of state policies is available here: