Septic System Construction

Jump to septic system sizing info

When constructing a septic system in Snohomish County, please be aware of the following rules and regulations:

  • A septic application approval is not a permit for the installation of a septic system. Permits for new structures or residences are not issued until the city or county building permit is issued. Note: If the building permit is withdrawn, revoked, or expires, the installation permit will no longer be valid.
  • When applying for a building permit, City or County Building Department officials may determine that your proposal be subject to the requirements of House Bill 2929, the Growth Management Act, Section 63, regarding the demonstration of an adequate and potable water supply.
  • If the Application for an Individual Water Supply Site Inspection is submitted concurrently with the Application for an Onsite Sewage Disposal Permit, the fee is less than if it is submitted on its own.

Septic Contractors

Septic professionals such as pumpers, installers, and monitoring and maintenance specialists receive individual certifications. Some septic contractors are certified in more than one area. 

Septic_Rectangle Buttons_Find a Contractor

Septic System Sizing

A licensed septic designer or professional engineer is needed to design and determine the size of your system. 


The number of bedrooms in your home can impact the size of your system. The Snohomish County Health Department’s Code outlines the policy on bedrooms. Potential bedrooms are considered bedrooms regardless of the placement of closets, windows and doors, or the width or location within the structure. 

For more information, you can review the Interpretation of Bedroom and Daily Flow Determination Policy.

Rooms that are considered potential bedrooms include, but are not limited to:

  • Bonus rooms
  • Dens
  • Hobby, sewing and exercise rooms
  • Lofts
  • Offices and studies
  • Unfinished rooms

The Snohomish County Health Department typically does not count rooms such as living rooms, family rooms, dining rooms, storage areas, closets, and utility rooms as potential bedrooms.

If your system is commercial, you need to discuss with a septic system designer how your system should be sized.