Snohomish County Septic Design


Site Evaluation

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Often referred to as a 'perc test', this investigation assesses septic system options under Snohomish Health District Code (Chapter 8 - On-Site Sewage Disposal).

The goal of the evaluation is to locate the deepest usable soils on the property.  This generally corresponds to the lowest system installation costs.

Our evaluation normally starts with a meeting to identify project goals, property boundaries and potential site constraints.  We then walk the site, evaluate vegetation, topography and drainage patterns and dig a series of soil log holes to assess usable soil depth and texture in potential drain field areas.  Next, we record hole details and location.  Finally we generate either on opinion letter or annotated sketch summarizing system options allowed under Snohomish Health District Chapter 8. 

Mitchell Septic's data collection is typically performed with a Total Station, which allows us to precisely layout system components to determine the maximum septic system capacity of the property.  This is critical on small lots and complex environments and provides better options on larger parcels.

The Snohomish Health District will accept and review soil log information as a stand alone submittal.  This option is not usually cost effective in the new construction permitting process.

Septic Design

Mitchell Septic generates a septic system design for submittal to the Shohomish County Health District for their review and approval.

An approved design is required to support a building permit.

There are separate design submittal and Installation Permit fees.

Alternative systems require a "Declaration of Covenant" signed, notarized and recorded at the Snohomish Auditor's Office.

Once approved, the design is valid for two years.


You request bids and select a Certified Installer from the Health District list.

There is a pre-construction conference with the Designer, Installer and Snohomish County official.

The Designer of Record is required to perform a final inspection and create an As-built.