Detached ADU - Determining Development Costs? - Portland, Oregon

9 Replies

Hi Everyone,

My first post here - I have been reading and studying the great resources on this site and learning a ton.  I have a single family rental in Portland that is cash flow positive, but not by a lot.  I ran it as a VRBO for a couple years and did well, but as my career got more intense I switched to longterm rental.

I'm researching adding a detached ADU to the lot to boost my returns. My livable square footage is about 1,800sf so that puts me at the limit of 800sf for the detached ADU assuming I am reading the code correctly. So I have several questions and I'm hoping the experts here can point me towards some good resources:

  1. What are typical development costs for a detached ADU (permits, etc.)? Any good websites that show examples/typicals on these?
  2. Where can I find a list of all costs to execute a detached ADU so that I can avoid surprises and overruns?
  3. Anyone have a slick proforma excel document for a detached ADU?
  4. I like the idea of container homes because of the "turnkey" nature and the fact that I could remove it at some future date to another site (or sell it) if/when I wanted to develop the lot to multifamily (which it is zoned for).  Are these allowed in Portland?  Anyone had any experience with these?
  5. Any advice on configurations (# bedrooms/baths) to maximize returns?

Thanks in advance for your help everyone!



Hi Steve,

You should reach out to Kol Peterson. He is a Portland resident that offers a site consultation and a wealth of knowledge, he's even published a book on the topic. He's partnered with earth advantage to teach us realtors and appraisers about ADUs offering those of us that take his class an ADU Specialist Designation.

Check out this website for examples for everything you could want to know about ADUs.  Costs, examples, builders, etc.

Good luck!

Steve -

Emily is spot on with the Looks like you are in the center of the ADU universe (Portland). Plenty of local news online regarding ADU's and the use of modulars.

Containers are an option.  There are some cool to crazy modular offerings that can be trucked in complete or assembled onsite (Perhaps Amazon soon).

It's also an interesting time in areas outside of Oregon. In CA, state legislators have passed a bundle of land use regulations to assist in the development and delivery of ADU's everywhere (almost).

There are also some very nice academic studies to help identify market demand drivers, and cost projections (Go Bears).

Looking forward to reading about your progress.

Good luck,


Steve, I am in almost the exact scenario as you.  I have a rental in Portland with positive cash flow but am very crious about next investment.  I am researching different options and very curious about adu on this property.  I also see we are both boaters as well.  Anyway would love to hear what you find out.  Ryan

Kol Peterson, as mentioned, is an excellent resource. You should also check out Dweller - They are doing some really cool things with prefab ADU units.

@Steve Green The City of Portland is really trying to incentivize more ADU developments so they've extended the site development cost waiver indefinitely. This basically saves you around $35k in fees. Not too shabby. 

It's definitely more cost-effective to convert a basement or garage to an ADU since you won't have to deal with a lot of the big expenses that come with new construction (like pouring a foundation) but for detached new construction I'd you can reference this website to get an idea of cost. 

I'll share what I know! We have a rental in Mt Scott that is a 2BD on the main house and we converted a garage in the back to an ADU ~300 sf.

1. Permits ran us about $2500. Dev costs are waived, and still should be. 

2. Seems very variable right? I'd treat it like any construction project though. 

3. I can share what w did, but sounds like you are looking at 800 sf, ours was 300. Not sure its of much use. 

5. Ours nets $1500 month on Airbnb for a studio. Hard to beat that, especially in outer SE. 

Hi everyone, thanks for all the great info you have shared on this post. I have been conducting research and there are some great detached design/build ADU companies as well as container home builders out there. The economics looks very good with either approach. I'm doing some further research on a multi-family approach before I commit to the ADU. Stay tuned and feel free to reach out to me with a PM anytime. Regards, Steve

Hey Steve - Curious if you made a decision on which direction to take? I'm looking to an ADU build and trying to collect information, experiences, recommendations, etc.


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