Strong Statewide ADU Reform Matrix

Legislative ADU reforms have been advancing at a remarkable pace in 2024/2025, making it challenging for even dedicated observers to track every development. For ADU advocates, this represents significant progress.

The trend of states reclaiming zoning authority from local jurisdictions marks a notable institutional shift. I trace this change back to the humble ADU, which catalyzed California’s statewide approach in 2016-2017. The concept is now spreading and evolving so rapidly that I predict the majority of states will implement preemptive statewide zoning measures to address housing issues—including ADUs—within the next year.

Why Statewide Reform Works

For local zoning reforms to succeed, several conditions must align: a pro-housing majority must be elected to city council, and planning staff must be committed to reversing decades of single-family-exclusive zoning. This combination is simply too difficult to achieve consistently, and I’ve concluded it’s unfair to expect local jurisdictions to shoulder this burden alone.

Even when political conditions favor housing reform, the policy implementation process requires enormous effort. Portland’s Residential Infill Project exemplifies this challenge—city staff devoted five years (2016-2021) and likely tens of thousands of hours to complete this crucial initiative.

Statewide reforms offer a stark contrast in efficiency, typically achieving policy changes within 0-2 years. Beyond speed, scale represents the fundamental advantage. Housing affordability operates as a regional issue, not a local one. Even if one city completely solved its housing crisis through innovative zoning and financing reforms, neighboring jurisdictions would remain unaffected, let alone addressing statewide housing shortages. In the absence of regional governments with statutory zoning authority, states represent the only viable mechanism for meaningful housing reform.

Observing Portland’s glacial zoning overhaul likely influenced Oregon state legislators to intervene through House Bill 2001 in 2019, efficiently extending similar “controversial” housing policies to cities across the state.

Analyzing Strong State ADU Reforms

Given the effectiveness of state-level ADU reforms, they deserve greater attention and analysis.

In partnership with the Mercatus Center, we have studied and cataloged the nation’s ten strong ADU zoning reforms, spanning Arizona, Arkansas, California, Colorado, Iowa, Maine, Massachusetts, Montana, Oregon, and Washington. Beyond this substantial number of states adopting strong reforms in the past year, intriguing variations have emerged within otherwise strong ADU legislation. These differences between states with strong reforms now capture my attention most—over time, we may systematically determine how specific zoning reform policy variations impact ADU adoption rates.

We excluded deeper policy analysis of the eight other states that have adopted only weak ADU reforms. Until primary barriers are addressed, meaningful ADU adoption remains unlikely, making further analysis of those states premature.

The Mercatus Center will publish our findings in September 2025. The paper’s essence can be understood through one comprehensive graphic, which is also available here as a high-resolution .pdf.

I was surprised to discover that Oregon, my home state, doesn’t compare favorably even against ADU reform newcomers like Arkansas, Iowa, and Montana. Come on, Oregon!

More seriously, fascinating policy fault lines have emerged, such as whether states have established dedicated statewide authorities to implement ADU reforms, and whether statutory authority supersedes homeowners association restrictions.

Moving forward, I’m eager to track these laws’ real-world outcomes. How many ADUs are actually being constructed as a result of these changes? Which detailed policies matter most for ADU production?

Come to think of it, we did not inquire about whether States are compelling jurisdictions to report their housing production metrics …we’ll have to add that one in next round.

About Kol Peterson

Kol is an ADU consultant, advocate and author of Backdoor Revolution: The Definitive Guide to ADU Development. Read more here: AccessoryDwellingStrategies.com and learn about building your own at BuildingAnADU.com. Email at Kol@accessorydwellingstrategies.com

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