Accessory Dwelling Units

Detached adu in Olympia neighborhood

What is an ADU?

Commonly referred to as a "mother-in-law apartment" or "granny flat", an Accessory Dwelling Unit (ADU) is an additional living unit with separate kitchen, sleeping area and bathroom facilities. ADUs can be attached to or detached from the primary single family residence and it is on the same lot. ADUs can be attached to the home or garage or built as a small, stand-alone, structure.

How to begin

The City strongly encourages anyone who wants to build an ADU to contact a planner to talk about the zoning and design review requirements early in the process. This will help to identify issues and pick options that will be viable for your property.

For example, every property is located in a zoning district. Each zoning district has requirements for where buildings can be located on the property, how much of the lot can be covered with buildings and impervious surfaces, and the maximum building height allowed.

Other requirements are directly related to the house on the property. The ADU must reflect the architectural style of the main house by using related building features (roof form and pitch, window patterns and proportions, or the use of matching or compatible façade materials and colors).

Additionally, the ADU must have a clearly defined entrance that is easily accessible from the street or the existing home. There must be a well-lighted, paved walkway to the entrance.

There is a limit of one ADU per lot on lots that have a single family detached home. Conversions of a portion of the existing home (such as converting a basement to an ADU) or adding on to an existing home to create an ADU will require building plans that are specific to the proposal. If you are considering a stand-alone ADU, you may want to take advantage of ADU plans the Cities of Olympia, Lacey, and Tumwater have available. Of course, for stand-alone ADUs, you can still choose to have custom plans created for your project.

Pre-Approved ADU plans are great – and they will save time and money during the review and permitting process. But there are site-specific requirements that need to be addressed before the permits will be issued. There are four pre-approved plans to choose from, each with exterior options to help ensure the ADU reflects the architectural character of the existing home. Contact a planner to learn about these requirements and to help determine which pre-approved plans could work for you.

Other considerations

  • While the City planner may be your first point of contact, depending on the proposal you may also need to talk with other staff about utility connections, stormwater, and building codes.
  • Fire Sprinklers will be required for a new ADU if the existing house has a fire sprinkler system.
  • Is the existing house on the historic register or in a Historic District? If yes, additional provisions may apply.
  • Is the existing house served by an on-site septic system for wastewater? If yes, it may be necessary to connect to the sewer system.
  • The ADU can often make use of the water and sewer connects from the street to the main house. Such connections have to be made in conformance with code requirements.
  • How will the stormwater from any new impervious surfaces (roof, walkways, patios, parking areas) be handled? This must meet code requirements and the information must be included in the permit application.
  • Will you provide additional parking for the ADU? It is not required but is allowed. A residential parking area is 8-feet by 16-feet. Parking areas must be paved and included in the impervious surface calculations.
  • Are there any wetlands, steep slopes, or other environmentally sensitive areas on or near the property? If yes, additional setbacks (buffers) and special reports may be required before a building permit can be issued.
  • A Site Plan with certain information is needed with permit application. Please see the Sample Residential Site Plan for help.
  • Consider you neighbors. Think about building placement and orientation, privacy issues, fencing, landscaping, and the location of any existing solar panels.
  • Reduced impact fees are required for ADUs and are charged at time of building permit issuance.
  • The ADU must meet all Building Code requirements, such as the minimum size requirements for habitable structures. Habitable rooms must be at least 70 square feet (6.5m2). If a room has a sloping ceiling, any part of that room with a ceiling below 5 feet in height is not considered as habitable.


Contact the Permitting Center Monday - Friday, 8 a.m. - 4 p.m.

By Phone: 360.753.8314
Via Email:
In Person: Olympia City Hall - 2nd Floor, 601 4th Avenue E