Accessory Dwelling Unit
See also: Garages, Neighborhood Residential Code
What Is It?
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.
SDCI will report all ADU permits to King County for sewer treatment capacity charges. During your application, you will need to fill out a form acknowledging those capacity charges.
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.
Pre-Approved DADU plans
The Office of Planning and Development (OPCD) maintains the ADUniverse website. The ADUniverse features a gallery of pre-approved DADU designs, a step-by-step guide to the ADU process, and a search tool to identify the feasibility of adding an ADU to your property. Created by local designers and architects, the pre-approved building plans offer a faster, easier, and more predictable permitting process for creating a DADU. In most cases, you can get your permit in just 2-6 weeks. See How To Apply for a Detached Accessory Dwelling Unit Using a Standard Plan for more information about the permitting process.
What Permits Do You Need?
Adding within an existing house. You need a construction addition / alteration permit.
Building a detached unit. You need a construction addition / alteration permit.
Legalizing an existing unit. You need a construction permit to establish use.
You may also need to apply for electrical service changes or new services from Seattle City Light.
Research the Code
- Accessory Dwelling Units, Seattle Municiple Code 23.42.022
- Neighborhood Residential Code
Associated Documents
- King County Sewage Treatment Capacity Charge - Residential Sewer Use Certification
- Release of Owner Occupancy Covenant for ADUs
- Tip 103, Site Plan Requirements
- Tip 106, General Standards for Plans and Drawings
- Tip 116A, Establishing an Attached Accessory Dwelling Unit (AADU)
- Tip 116B, Establishing a Detached Accessory Dwelling Unit (DADU)
- Tip 217, How to Legalize a Use Not Established by Permit
- Tip 606, Illegal Dwelling Units
- DR 7-83, Determining the Existence of a Dwelling Unit for the Purpose of Code Enforcement