Accessory Dwelling Units (ADUs) are an affordable type of infill housing. They are small, include a bedroom, kitchen and bathroom, and can be either attached or detached from the main house on the property.


Accessory Dwelling Units (ADUs) are a separate additional living unit, including separate kitchen, sleeping, and bathroom facilities, attached or detached from the primary residential unit, on a single-family lot. ADUs are usually subordinate in size, location, and appearance to the primary unit.

One type of accessory dwelling unit is an attached unit, contained within a single-family home, known variously as "mother-in-law apartments," "accessory apartments," or "second units." Accessory apartments usually involve the renovation of a garage, basement, attached shed, or similar space in a single-family home.

There are also detached "accessory cottages" or "echo homes" (an acronym for "elder cottage housing opportunities"), which are structurally independent from the primary residence. These units are often constructed or installed to provide housing for elderly parents being cared for by their adult children.

ADUs can provide:

  • A broader range of affordable and energy-efficient housing
  • Neighborhood friendly infill development that makes efficient use of existing housing stock and infrastructure, increasing density while maintaining Olympia’s small town feel
  • Expanded housing choices for a variety of household configurations
  • A means for residents, particularly seniors, single parents and families with grown children, to remain in their homes and neighborhoods
  • Opportunities for homeowners to obtain extra income, security, companionship and services
  • Opportunities for innovation with small home design and green building
  • A legitimate pathway for an activity that is often done without permits
    This blog was created as an online resource for information on ADUs and other uses of small home design by Cameron Hostetter for an independent learning contract at the Evergreen State College in Olympia, WA.