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House or building remodels or expansions

Find the permitting steps for your remodels or expansions on septic systems.

OSS permitting review timeline

➜ OSS permitting review timeline

The OSS Permitting Program's current review time is approximately 12 weeks. This is due to staffing turnaround and a very high number of OSS applications submitted this spring. Please find your application on the Status Lookup for project-specific status updates. We appreciate your patience as we work diligently to review your applications as quickly as possible.

Steps for house or building remodel or expansions

  • You may apply for this review prior to or at the same time as the Building Department's review of your application.
  • While preparing your application, it is important to accurately describe the location of all septic system components on your site plan. Search and download a drawing of your septic system (as-built drawing).
  • Each application is reviewed by an inspector and receives a site visit. Examples the inspector are looking for includes:

    • Age of septic system
    • Condition of system
    • Type of system
    • Location of system as compared to area of remodel
    • Presence of a reserve drainfield area
    • Size of remodel
      • existing square footage vs. square footage being added
    • Use of addition
      • are bedrooms being added? If so, call the King County Septic Expert first! 206-477-8177
      • number of bedrooms equals occupancy equals size of septic system
    • Water source
      • a drinking water well requires certain setbacks to structures
      • structures must be served by an approved water source
    • As-built and past permit history
  • Due to the scope of your remodel and condition, location and age of your septic system, you may be required to have the septic system evaluated by a licensed septic professional. The inspector will notify you in writing what this evaluation must include and what type of septic professional must be contacted.
  • You may also be required to upgrade or replace your septic system. For example, a house with an older septic system and your proposal is to tear the house down to the foundation and rebuild, it would require a new septic system. Call the King County Septic Expert 206-477-8177 to ask about your potential project.
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