Accessory Dwelling Unit

This section explains what an Accessory Dwelling Unit (ADU) is and the steps to follow to create one. Design considerations, getting approval from your city (zoning/permitting), and selecting an architect and contractor are discussed.

An Accessory Dwelling Unit is an extra living unit on your property, complete with kitchen, bathroom and sleeping facilities. Subject to local regulations, ADUs may be located either inside, attached to, or detached from the primary home on your property. Another common term for ADU is ‘Mother in Law’ apartment. Read Seattle Times ADU article 'Both sides can benefit in 'Mother-in-law arrangement'.

What Are The Benefits Of Owning An ADU?

  • Extra Housing
    Meet a family need or have a tenant living nearby.

  • Companionship, Security, and Home Help
    Others to keep you company, look after your well-being, or help out around the house.

  • First Time Homebuyers
    Possibly qualify for a larger home loan, have extra income, or more choice in using your home.

  • Income
    Extra cash in your pocket for mortgage payments, home repairs or anything else you need.

What's My Next Step?

Keep in mind, in the last few years, many cities have changed their regulations to allow ADUs in single family neighborhoods. If you find that the idea of owning an ADU sounds interesting, and you would like to explore it in more detail, you will find a lot of tips and information about ADUs in this section and links to other websites that can assist you with creating or operating an ADU. Take your time to browse it, and feel free to download or print the parts that interest you. Enjoy!

What is the ADU Homeowner Packet?

There are a number of steps involved and decisions to make in developing and operating an ADU. We have spent a good amount of time learning about what is involved from homeowners who have already been through the process, and know (first-hand) what it is like. Generally speaking, an ADU project will be similar to a remodel or addition to your home. This packet is organized to touch on the areas that we think you will be most interested in. Remember, we have tried to cover a broad variety of situations in each of the topic areas, so while there is a lot of information, not all the steps or information may apply to your project. We have tried to write sections in a manner that explains steps in detail for individuals with little or no experience with construction projects. Therefore, individuals who have already been through a construction project may find some of the sections 'wordy' or elementary to them. We believe the information in this packet will give you a good head start in the initial planning and organization of your project, as well as an important reference as your project progresses. As this program continues, we hope its users will provide us with additional insights and tips, which we can pass along to other users.

Please Note: that while this packet attempts to provide you with examples of people or businesses you can call to help you with various aspects of your project, these are only examples and are not meant to be either an exclusive list of your choices or options, nor necessarily the preferred or recommended options for you to use.

Resource Assistants

A resource center has been established at both ARCH and the City of Seattle to assist you with general questions you may have about ADUs or the information provided in this ADU Homeowner Packet. If the questions require a more technical response, the resource assistants may refer you to a more appropriate person or agency for response.

Please feel free to call this individual anytime between the hours of 8:00 am to 5:00 pm, Monday through Friday.

If you live in a city in East King County, or in the unincorporated area around one of the East King County cities, contact King County or your city's permit department. 

If you live in the City of Seattle, or in the unincorporated area around Seattle, contact the City of Seattle Department of Planning and Development or visit their website here.

This ADU HomeOwner Packet, a variety of other housing and ADU information, and links to related information can also be found on the City of Seattle Website.