Seattle, WA Accessory Structures: Tiny Homes (2026)

Accessory Dwelling Unit — An accessory dwelling unit (ADU) is a separate living space within a house or on the same property as an existing or proposed house. These units may not be subdivided from the main house and are not legal unless they have been established through a permit process. A legally permitted unit in the home is called an attached accessory dwelling unit (AADU). A legally permitted unit on the property (but not within the home) is called a detached accessory dwelling unit (DADU). Accessory dwelling units are restricted in size and generally count toward density limits. They must comply with the property zone's height and location standards. While some property may be large enough to have four or more units, only two of them may be ADUs. Certain standards do not apply to accessory dwelling units such as parking, street improvements, and mandatory housing affordability contributions. An ADU must meet current Seattle residential, building, mechanical, electrical, energy, land use, environmentally critical areas, and shorelines code standards. [...] Note: Tiny houses on wheels are treated like camper trailers. You cannot live in a tiny house on wheels (or similar equipment such as RVs and boats) on lots in Seattle city limits. If your tiny house has wheels you need to follow parking rules for large vehicles. Tiny houses, with foundations, are considered DADUs.