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Backdoor Revolution- Part II- Informal ADUs

This is part II in a series called ‘Backdoor Revolution’, written to summarize some interesing things I learned while writing a new book about ADUs.

 Post  Title
Part I Origin
Part II Informal ADUs
Part III Why Fostering ADUs is Different than Fostering other Housing Types
Part IV Top Ten Reasons Why Basement Conversion ADUs and Detached ADUs Are Like Chalk and Cheese
Part V Why Vancouver, BC has Done a Lot of ADU-Related Things Right

One significant part of my book research was about informal ADUs. While permitted ADUs are still fairly rare, informal ADU (a.k.a. unpermitted ADUs) are not.

The research on this topic is historically challenging to conduct.

  • Informal ADUs are discreet. They’re commonly found within basements, garages, and in other permitted structures.
  • Informal ADU owners may not necessarily like to seek attention about the existence of these units.
  • The line between an informal ADU and other habitable living spaces is quite blurry- and the definitions are not firmly established. i.e. One’s person’s informal ADU may be someone else’s mother in-law suite.
  • People who have an informal ADU may not neccessarily think of it as being one–they just view it as an spare unit or even just an extra room. 

By wading through dusty research journals, I was eventually able to read through more than ten studies of informal ADUs.

These studies all point to a remarkable percentage of informal ADUs mixed in among conventional housing stock. In the areas that were studied, generally, 10%-80% of all residential housing units also had informal ADUs.

This points clearly to the market need for the type of housing that ADUs, informal or permitted, represent.

Once politicians and officials realize that there are bountiful informal ADUs in their city, they must inevitably confront the tricky ethical question of whether to try to eliminate them.

If they do, they are removing some of their most affordable housing stock…likely in the midst a housing crisis. If they don’t, they are turning a blind eye to unsafe housing conditions.

I don’t envy those who must deal with this tricky question. But, I think it’s actually the wrong question to ask.

Instead, we should informal ADU as:

  1. a simple expression of market demand
  2. clear evidence that there are regulatory barriers.

There is an extremely succinct prescription for jurisdictions that realize that they have a lot of informal ADUs:

Make the regulations for permitted ADUs more flexible.

Granted, this prescription won’t completely resolve the issue of legacy informal ADUs, but over time, it will chip away at the problem.

I believe that this is happening in Portland. I would guess that there is a lower percentage of informal ADUs being developed today than there was in 2009 before Portland’s system development charge waiver. It’s become common in Portland to develop them legally; there’s few reasons to bother building an informal ADU, and everyone else is building with permits–it’s the working standard to get permits at this point. 

Recent data from City of Los Angeles shows that most of the recent spate ADUs are actually legalizations of informal ADUs (captured under the “conversions” category in the chart below), not new construction (captured as “backyard homes” below).

L.A. has been a poster child of informal garage-conversion ADUs for decades. This was documented in a couple studies in the 1980s, and this research made the paper. 

For policy makers, this recent rush of ADUs legalizations in L.A., should not be confused with an ADU legalization “amnesty program”. L.A. does not have an amnesty program.

Indeed, one of my findings is that amnesty programs have not worked. Sure, amnesty programs sound charming, and many cities have tried them.

But historically, these programs simply don’t compel many people to legalize informal ADUs–the risk of an individual’s ADU legalization attempt going awry during the legalization process is far too great, and the reward of legalization is far too small.

What likely happened in Los Angeles is that homeowners with informal ADUs started flooding the permitting office gates using California’s flexible development regulations, which were prompted by California’s 2017 state ADU legislation.

Homeowners rushed through the regulatory gates before they would miss their window of opportunity to legalize their units. The City is entitled to enact its own regulations if it would like to do so, and those regulations may ultimately be far less flexible than the State standard currently being used by the City.

In writing Backdoor Revolution, I discovered that informal ADUs have a lot of lessons to teach us. The book is now available on the book’s home page

About Kol Peterson

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

5 comments on “Backdoor Revolution- Part II- Informal ADUs

  1. John DL Arendsen
    January 11, 2018

    Hi Kol, I read and very much liked this article. I’ll be passing it along to some of our local jurisdictions. Many if not most are dealing with this issue.

  2. Pingback: Backdoor Revolution- Part III- Why Fostering ADUs is Different than Fostering other Housing Types | Accessory Dwellings

  3. Pingback: Backdoor Revolution – Part I – Origin | Accessory Dwellings

  4. Pingback: Backdoor Revolution- Part IV- Top Ten Reasons Why Basement Conversion ADUs and Detached ADUs Are Like Chalk and Cheese | Accessory Dwellings

  5. Pingback: Backdoor Revolution- Part V- Why Vancouver, BC has Done a Lot of ADU-Related Things Right | Accessory Dwellings

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