Accessory Dwellings

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Owner-Builder ADU by Kat and Dad

Quick Facts

Setting: suburban

Neighborhood: Buena Vista, Walnut Creek, CA

Type: detached

Use: rental

Square Footage: 1,200

Year Built: 2020

Owners: Kat and family

Designer: self

Builder: self

Total Cost: $200,000

When we saw this fixer-upper on a half-acre plot of land, I knew right there and then: we were going to buy it. That was back in 2018, and my father started plotting the renovation strategy while exploring the property. One of its best features was the enormous unoccupied front yard, which seemed like a good spot for a second house. We were about to discover the secret world of ADUs and embark on an exciting journey of building one.


First pleasant surprise were the changes to California housing legislation in 2017, making it easier to build ADUs. Second, I learned that our new property belonged to an unincorporated subdivision of Contra Costa County, allowing the maximum ADU size in the state: 1,200 sq ft (unlike the City of Walnut Creek with 800 sq ft). Then came another piece of good news: there was enough space in the front yard to observe the zoning ordinance and setbacks. It was my mother’s idea to build specifically in the front yard to utilize the unused hilly area and provide easy access to ADU by the tenants, while keeping the privacy of our backyard. The hill complicated the design and build process, but I believe it will ultimately pay off.

After getting the ADU packet from the building department, I started gathering the required documents. We considered all kinds of designs: from prefab and panelized kits to steel and wood framing. Eventually, we landed on the traditional stick frame for its price and ease of build: the infamous hill did not allow for any standard designs so we had to build custom. Having found remotely similar plans online, we took them to a design firm to turn our ADU concept into architectural and structural plans. They listened attentively and delivered exactly what we had in mind: split-level, 3 bedroom, 2 bath unit with a large family room and an open-space kitchen. The mock designs helped us visualize exactly what we’re building, minus the exterior finishes that needed to match the existing house in white and blue:

Here are the floor plans in PDF and AutoCad formats for anyone interested.


With all the documents in hand, I filed a permit application in November 2018. Because of my travels afterwards, it took longer than usual to get approved, but after 7 months of back and forths I got a shiny new permit in June 2019. It wasn’t cheap – $16.5k – but I knew we’d reap significant savings when building ourselves. Applying as owner-builders taught us everything we needed to know from properly reading the plans to successfully passing inspections. Besides, the building department was beyond helpful and understanding: they told me exactly what the structural engineer was missing, how we should approach the building process, and where to find help. I had the building code hotline on speed dial and got to know many of the employees by name. Any myths that the building office is there to make my life difficult were shattered: quite the opposite, it was there to help.

Some of not so exciting surprises came from the public utility agencies: my water and sewer connections ended up costing a fortune. $36k was the price tag for a separate water meter required for ADU and $5k was the sewer lateral connection. Overall, permits added up to $60k or 30% of my total budget.


With the permit in hand, all the building fun began. My father never built from the ground up in California, but he’s had a lot of experience in Ukraine. The two parts of the job he was absolutely uncomfortable with were foundation and electrical, so I started looking for subcontractors. Everything else was taken care of by him and yours truly: we broke the ground, did our own excavation and compaction, and waited for the foundation to be approved before we could start framing. That part didn’t begin until March 2020, due to some foundation design changes and abnormally rainy winter. When COVID-19 pandemic hit, we were grateful for construction stores to remain open and completed the bulk of the work during the 3 months of shelter in place. Framing was my favorite part: it was fascinating to see the structure take shape, piece by piece. We did most of the things very old school: my father didn’t let me buy a nail guy and ended up doing it all by hammer. We also opted for roof rafters instead of trusses, which helped to keep the budget down. A key factor during this phase was working with an experienced framing consultant that I found through Craigslist: without doing any physical work, hwe would come out on site to confirm we were doing things right.

My part in the process was mostly in ordering materials, coordinating inspections, and figuring out the budget. For the materials, we simplified things by ordering from 2 stores only: local lumber yard for framing and hardware, and Home Depot for everything else, from insulation and doors to lamps and kitchen appliances. I also took care of subcontracting foundation and electrical work, which seemed like a Herculean effort: finding professionals who were available and charged reasonable prices took quite a while. I am still puzzled by the fact that my foundation quotes ranged from $18k to $140k, and electrical – $3.5k to $24k. With 95% of the project complete, we’ve spent close to $200k so far:

The detailed budget breakdown is available in this spreadsheet.

After learning the ropes in the process, I could not keep all the information in my head and needed to share. This inspired me to start a series of blog posts about my ADU journey, which was later accompanied by analytics on the state of ADU in California. The purpose of the entire blog is to inspire fellow homeowners to build more ADUs, no matter if it’s 100% DIY or completely turnkey. Whatever your purpose is – to earn rental income, have grown up children live nearby, or retire in place – ADUs can be a perfect solution for that. Besides, they are meant to address the housing affordability all over the country, and I’m looking forward to the day we can put the term “housing crisis” to rest.

Please reach out with any questions, comments, ideas – happy to share all I know about ADUs to date. I also promise to post a video walkthrough of the unit once it’s finaled and ready for prime time!

~ Kat

6 comments on “Owner-Builder ADU by Kat and Dad

  1. Andy P
    September 30, 2020

    That looks really sharp Kat. I’m jealous you were able to build it so large. Having built two ADUs of my own in Charlotte, NC (720 sq ft and 800 sq ft) I wanted to weigh in on how I handled the water and sewer connections to avoid the steep tap fees Perhaps this suggestion can help others. In Charlotte, it is ok to use a sump pump connected to the main house’s cleanout line. (Contractor installed a Y-adapter of sorts, so I still have the cleanout.) We intercepted the water line in the front yard and trenched the yard to accommodate water and sewer connections. We rent all of our units out on our own website and through AirBNB and have helped MANY MANY folks relocate here from CA and NY. Mindboggling the exodus from those two states since January.

  2. Suzanne
    January 12, 2021

    Hello Kat,
    I love your site.
    My husband and I own a duplex that has a three car garage. We want to turn the garage into two 1 bedroom ADU units. When we call the city for permit costs for water and sewer permits, or the MID (the electric company), everybody seems to say we are fine and don’t need to add a new meter or sewer hookups, that we can hookup to the existing box and lines. They are not quite sure that the fees quoted are the only fees we are guaranteed to have to pay. We have asked the city permit department:what else do we need to start the build besides a blueprint? Our fear is that we’re going to be in the middle of the project and find out the staff people we talk to are incorrect, halting the construction or costing us thousands in additional fees/permits/time wasted because they were wrong with their answers.

  3. Peter
    February 2, 2021

    Hi Kat, I am considering buying a new home with an attached ADU in the Walnut Creek area. What are the pros and cons of having a tenant living on your property? Thanks.

  4. Mike
    March 16, 2021

    Did you have to install a new sewer lateral or did they allow you to install the ADU sewer to your house sewer

  5. Eduardo
    May 2, 2022

    What a great project! I’ve been interested in doing an owner build also. Maybe something smaller, but I’m just curious. Do you know around how much value did this adu add to the overall price of the property? thanks!

Comments are closed.


This entry was posted on September 30, 2020 by in 800 SF or Larger, Detached.
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