Accessory Dwelling Units (ADUs)


An ADU is a self-contained apartment in an owner occupied single-family home or lot

that is either attached to the principal dwelling or in a separate structure on the same property.

Allowing ADUs - also called granny flats, mother-in-law apartments or casitas – offer a means to convert oversized houses and garages, or add a second small unit on a lot.   They are a relatively simple way to use existing housing stock to increase affordable housing, by sidestepping some building code details that are unreasonably expensive applied to existing buildings.

  • They offer particular benefits in resort communities for caretaker housing and can lower insurance rates for often-empty second homes.
  • Restrictions may be needed in resort locations to prevent their conversion to short-term rentals or motel units, e.g. a $500/month rent restriction
  • and require full time residency.

  • Exempting ADUs from standard building code requirements for duplexes would help to keep them affordable. Firewall and sound separation, for instance, are expensive to retrofit into existing houses.

  • Requiring upgrade of water and energy performance of the buildings so that demand on utilities does not increase can help justify waiving Systems Development Charges.  If a building is using less energy, water, and producing less sewage with two families than with one, it is a good argument for fee waivers as well as improvement in efficiency.

  • A 1989 survey of 47 communities found that communities can expect to get approximately one ADU per 1,000 single family homes per year.  In ten years, that would produce 1 ADU per 100 existing homes.

  • Portland, Seattle, and Cannon Beach already have ADU ordinances tailored to specific community needs.   Neahkahnie (under the jurisdiction of Tillamook County) has 10 units at present and allows them as a conditional use, which is expensive to obtain. 


  • Detached Accessory Dwelling Units, Director’s Report, Feb 17, 2004, DPD, City of Seattle  - While focused on detached ADUs, this contains a good report on Seattle’s 10 year experience with ADUs – neighborhood fears vs. what really happened, how may have been developed, their geographic spread, etc.  Good experience report.
  • Establishing an Accessory Dwelling Unit (ADU)   (downloadable PDF File)  - This is Seattle’s client assistance homeowner packet explaining the regulations, process, and permits for establishing an ADU.  Application form included.



  • The City of Santa Cruz has technical manuals, regulations, etc. on their website  (   Plus the Santa Cruz Credit Union has a loan program specific to ADUs (


Assist local jurisdictions in developing, implementing and managing ADU ordinances appropriate to a tourist economy.