California and New York are blessed with incredible natural and economic advantages, but they’re hemorrhaging people. ¹ They’re driving people away by strangling their housing markets. People still want to live there. You can tell by the sky-high housing prices. But those same prices are forcing people to leave. What can be done?

What we really need to do is for states to exercise their land use sovereignty and legalize housing in general, sweeping away all the local restrictions, extortionate shakedowns, and endless byzantine procedural obstacles. But in California the most ambitious attempts to change the rules statewide have failed in recent years, and New York City is keeping its costly and pointless parking mandates. The YIMBY movement is making progress, just maddeningly slowly.

But there’s one really bright spot. In 2017, California really did essentially abolish single family zoning statewide. SB 1069 mandated “ministerial approval” for accessory dwelling units (ADUs), and a stream of legislative efforts since then have methodically whack-a-moled the barriers that our NIMBY localities tried to impose on them. The restoration of homeowner freedom to at least build these things has advanced pretty far, and the result is a lot of new homes. Something like a hundred thousand ADUs have been added statewide since 2016 ² .

And 13 other states have followed suit. Roughly 100 million Americans now live in jurisdictions that broadly favor the construction of these things.

That said, I’ve had some conversations recently in which I tried to explain that ADUs are really hard for cities to stop, and have been surprised to find that people kinda just…don’t believe me! “Oh, I don’t think those are allowed.” “The permits must be a nightmare.” Many people still really just don’t know this is an option!

I got inspired to make a contribution to the cause of housing abundance, and built something to help share the good news that ADUs really are an option.

I started close to home with some analysis of the situation for Oakland and Berkeley, trying to estimate how many homeowners could reasonably build a detached ADU. I started with all the properties in each city, ³ the existing building footprints, and a model of the terrain, then excluded the areas too close to the lot lines and the existing buildings. Here’s what I found:

In Oakland, 20,273 of the 56,737 single-family properties (36%) have enough open space for a detached ADU. If all these opportunities were realized, Oakland could house over fifty thousand more people, increasing its population by about 12% from its current ~436k residents.

In Berkeley, the lots are bigger. 5,794 of the 9,546 single-family properties (61%) seem to have enough open space for a detached ADU. If all these were built, Berkeley could house over 15,000 more people in ADUs alone, increasing its population by about 13% from its current ~119k residents.

These are rough estimates. Some are not actually realistic due to, for example, mature trees. Others are a mirage because they already happened! On the other hand, garage conversion opportunities and multi-family situations where multiple ADUs could be added are omitted. But it seems very clear that if the owners of all the properties who could build an ADU did so, there would be many new homes for people to live in. I want to encourage them to act, so to that end, announcing:

ADU Opportunity Center

If you live in a single-family home in Oakland or Berkeley, I probably have a detailed ADU opportunity report for you. But you can enter any US address to get some info on your state’s rules and how well an ADU would fit on your property.

If you’re a homeowner in California or one of the other 13 states with ADU-friendly laws, you might be sitting on an opportunity to create more housing while generating rental income, or providing a home for a friend or loved one. Check your property’s potential at https://aduopportunity.org. And share with anyone who might be interested! Help build our way out of this crisis, one ADU at a time.