Mercer Island promotes accessory dwelling units through its zoning code. Faced with high housing costs and limited developable land, ADUs provided an opportunity for Mercer Island to expand its housing stock and increase affordability without changing the character of existing neighborhoods. Similar to Santa Cruz, ADUs on Mercer Island are allowed by-right in all single-family residential zones and are subject to a series of restrictions. The property owner is required to live in either the principal unit or the ADU. Units must be a minimum of 220 square feet and a maximum of 900 square feet. Most single-family homes are required to have at least three parking spaces, including two covered spaces, one of which must be available for the tenant of the ADU.

Within the first five years of its adoption, Mercer Island’s ADU ordinance had encouraged the development of over 150 ADUs on an island of 24,000 residents. While there has not been a recent study of ADU affordability on the island, a 2000 study by King County, Wash., found that the average rent of a Mercer Island ADU was affordable to households earning 60 percent of median income.

For More Info:

Shannon Restall, City of Mercer Island Development Services – Building & Planning
Email: [email protected]