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Septic systems treat wastewater when homes and buildings are not connected to public sewer systems. A septic system is also referred to as an "on-site sewage system" (OSS) in official documents, forms and applications.

Whether a property is served by a septic system vs. a public sewer system affects the permit process.

Building permits. For lots not served by sewers, an approved septic design from the Department of Public Health - Seattle & King County is required prior to submitting a building permit application. Public Health staff also review applications for all remodels, additions, and detached structures on any land served by a septic system, ensuring that the system is adequate and will not be impacted by the new construction.

Land use proposals. Public Health is required to review subdivision proposals when the properties are going to be served by septic systems. Availability of water supply is included in the review.

See Related building permits for more information about plumbing and on-site septic system permits, or visit Public Health's Web site at