What is an Accessory Dwelling Unit? And can I build one in Washoe County?

If you haven’t heard yet, you will soon: Amazon just invested millions into a prefab homebuilder startup called Plant Prefab.

I woke up this morning and saw the news of Amazon’s investment all over the internet…

While most of the news stories focused on Amazon’s investment as an avenue for Alexa and smart home technology expansion, I was more interested in the prefab homes (prefabricated) offered by Plant Prefab. Plant Prefab offers a number of different models, as well as custom options.

One model that caught my eye is their AD1 Accessary Dwelling Unit, or ADU. This 400-sq.ft., one-bedroom, one-bath ADU has a base starting price under $100,000.00, and can be used as a stand alone “tiny house“, or as a guest house or “granny flat.”

I think these are pretty cool, and can see a whole host of uses for them: the aforementioned guest house; kids clubhouse; AirBnB rental; etc. Intrigued, the first question that came to mind was, “Am I allowed to install one of these ADUs on my property?”

So I visited the Washoe County Planning and Building Division to get some answers. The short answer is, it depends.

Washoe County, Nev. defines a “detached accessory dwelling unit”, or DAD, as a dwelling unit on the same lot as the main dwelling unit, but which is physically separated from the main dwelling unit. And there are a whole host of requirements for which properties may or may not have one installed — See Washoe County’s Planning and Development information on Detached Accessory Dwellings for the scoop.

From the linked Washoe County Detached Accessory Dwellings info above:

Can I build a DAD?

  1. How large is the property?
  2. a. Under 12,000 sq. ft. (.28 acres) – No, not allowed
    b. 12,000 sq. ft. or more – may be permissible
    c. Located in Incline Village or Crystal Bay, minimum of one acre required
    d. If on Septic, minimum of 2-acres required (for a second septic system –, this is a Health District requirement for the second kitchen) For more information see: https://www.washoecounty.us/health/regulations.php

  3. What is the regulatory zone of the property? [Ed. note: you may look up the zoning for your property on the Washoe County Assessor’s site.]
  4. a. Medium Density Suburban (MDS) – Special Use Permit required before applying for building permit
    b. Low Density Suburban (LDS), High Density Rural (HDR), Medium Density Rural (MDR) or Low Density Rural (LDR) – Detached Accessory Dwelling Administrative Review (DADAR) required before applying for building permit
    c. General Rural (GR) or General Rural Agriculture (GRA) – Allowed use, Building Permit required
    d. All other regulatory zones – No, not allowed

  5. How large can the accessory dwelling be?
  6. a. MDS – 50% of living area of main house, up to 800 sq. ft., whichever is less
    b. LDS, HDR, MDR, HDR or GR – 50% of living area of main house, up to 1,500 sq. ft., whichever is less
    c. GRA – no size limit

[Note: rules and regulations governing detached ADUs vary greatly across states, counties and municipalities, so your milages may vary.]

Unfortunately, I learned that my lot size is not large enough to allow a detached dwelling unit. 🙁

How about yours? Would love to hear your thoughts.


  1. Vale Trujillo

    This is great information Guy, thank you. I had this in the back of my mind, having just over an acre with lots of unused space. The second septic/size requirement is a real letdown though! I wonder if some of small rentals I see out in Washoe Valley on other larger properties are in compliance.

  2. Guy Johnson

    Thank you, Vale. The second septic/size requirement is what eliminated me, as well.

  3. Vale Trujillo

    I wonder if there are any kind of exceptions for the number of residents in the primary residence?

  4. Guy Johnson

    Vale, give the Planning Division ( 775.328.6100) a call and let us know what you find out.

  5. Gary

    I don’t think there are really any exceptions here. 1 acre and an accessory dwelling – needs 2 septic systems. Doesn’t matter how many people are living in unit. I think the extra septic system is just a trap to catch people – kind of like a fishnet – And it obviously works.

  6. Ellen Miller

    We added an in-law unit for my mil when we purchased our home. We were able to avoid the 2nd septic by attaching it to our home, hence eliminating the free standing element. I believe the current septic tank size was also a consideration. The unit is private with a separate exterior entrance. Fortunately our home was conducive to adding it with a garage underneath as well.

  7. Guy Johnson

    Hello Ellen,

    Thank you for sharing your first-hand experience with the readers. Much appreciated.

    – Guy

  8. LH

    Thank you Ellen for providing your insight on attaching a 2nd dwelling to the main house. I think that may be the route I take as well.

  9. Morgan

    I recently bought 10 acres in north red rock area can I instal a prefab accessory dwelling to live in? Without having a primary residence. I’m totally off grid

  10. Guy Johnson

    Hello Morgan. Thank you for your question. I would recommend starting with the Washoe County Planning and Building Division to get up-to-date answers.

  11. Marilyn

    Hi , I am wondering if I could add a prefab dwelling to my property in Sparks. We do not have C C R ‘s . My daughter is struggling to pay rent and all the bills with her two small children. Our home is 1616 square feet, with regular city sewer. We already have a big concrete pad on the side of our house ?

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