The septic system rebate program is available throughout all of King County (not just unincorporated areas), however there is an income eligibility requirement for properties outside of Vashon-Maury Island and the Poverty Bay Shellfish Protection District. Businesses are eligible if they are a small business (the simple definition is in the application).

The rebate program website has an eligibility map you can add an address to and find out what rebates are available for that property.