Issaquah, WA Accessory Structures: ADU Rules (2026)

Land Use Code § 18.500.090 applies to every accessory dwelling unit in Issaquah, whether it sits inside the main house or stands apart from it. An application needs Level 1 review under IMC 18.204.020, and a nonconforming structure can be converted into an ADU. The lot must meet the minimum lot size required for the principal unit under Chapter 18.404 IMC, Form and Intensity. An ADU tied to a detached single-family structure does not count toward lot density, while an ADU tied to middle housing or multifamily housing does count toward unit lot density.

Number and size: Two ADUs, in any combination of attached or detached, can be permitted for one single-family detached residence or with middle housing. The code's own example is one ADU in the dwelling unit and one above the garage. Legally nonconforming lots can also have up to two ADUs but must comply with Chapter 18.214 IMC. Each ADU is limited to 1,000 square feet, excluding nonliving areas such as garage area, storage sheds, or decks. The principal unit must have a gross floor area at least 25 percent greater than the largest ADU on the lot or within the development. An ADU inside a principal unit, or one added to an existing accessory structure, can exceed the size limit when the conversion needs no exterior expansion beyond minimal alteration for a functional separate residence.

Design: ADUs follow the height required for the principal unit. A four-foot-wide walkway must run from the address side of the right-of-way or driveway to the ADU primary entrance. New ADUs are not subject to rear yard setbacks where the rear yard abuts an alley, unless the city routinely plows snow on the public alley. Adequate on-site parking is required for the house and the ADU under Chapter 18.604 IMC, and tandem parking counts. The ADU needs a separate address, and the ADU or the main residence can host a home business approved under IMC 18.502.120.

Existing units: If an ADU was created without a finalized building permit, the City requires a new building permit to confirm the structure poses no hazard and meets the section and the Building Code. A removed ADU can be reinstated if the building official finds it sound on inspection. Utility connections are reviewed, and the ADU can share the principal dwelling's connection and meter when fixture count and fire flow demands are met. The City cannot prohibit selling a condominium unit separately solely because it began as an ADU.