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Backdoor Revolution- Part IV- Top Ten Reasons Why Basement Conversion ADUs and Detached ADUs Are Like Chalk and Cheese

This is Part IV of a blog post 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

Basement ADUs and detached ADUs are totally different creatures. 

As I began to research and write my book about ADUs of all types, I strongly considered writing a totally separate book about basement ADUs. I ultimately decided to capture basement ADUs along with other structural forms in the same ADU book.

A discrete basement ADU. The staircase entry passage begins just beyond the pedestrian gate visible on the right side of this image.

Here’s a top ten list of ways that basement ADUs differ significantly from detached ADUs.

  1. Basement ADUs are built within an existing building envelope. They are more hidden than detached ADUs. It’s easier to be discrete and fluid about how a single dwelling structure that includes a basement is partitioned and used by its occupants. It’s fairly common to see basement apartments and suites that aren’t officially ADUs.
  2. The building code issues that pertain to basement ADUs deal with firewall separation, sound separation, and other utility separation issues in different ways than detached ADUs.
  3. It’s common to have dedicated entries to a basement ADU, but it’s also possible to have a shared entry and a common utility room in the basement for both units.
  4. The design considerations for basement ADUs vary from a detached ADU.
    1. Basements are partly or mostly subterranean. Their temperature is naturally regulated by the earth that surrounds them. They typically don’t need AC.
    2. They’re usually on a single level.
    3. They usually have ceiling height constraints to contend with.
    4. They always have egress windows.
  5. They’re generally far cheaper than detached ADUs to build.
  6. Some Cities only allow attached ADUs such as basement conversions, while other cities only allow detached ADUs.
  7. Municipal permits for structural conversions such as basement ADU renovations are often tracked differently than detached ADUs, which makes it challenging to conduct basement conversion ADU research on public permit databases.
  8. There are different financing options available because they are attached and converted from an existing structure, as opposed to detached new construction.  
  9. They’ve valued by owners, appraisers, and tax assessors differently than detached ADUs.
  10. Cities commonly call basement ADUs and detached ADUs by totally different names!

Backdoor Revolution lays claim to the largest list ever compiled of synonyms for ADUs. And, that’s just in an appendix!

Basement conversions ADUs are just one of six common types of ADU structural forms found in the US. Backdoor Revolution explains and differentiates the different structural forms of ADUs and helps distinguish these differences so that both homeowners and Cities can think strategically about ADUs built within different structural forms.

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


This entry was posted on January 18, 2018 by in Uncategorized and tagged , .
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