Sizing Your Septic System

When sizing conventional and alternative on-site sewage disposal systems for single-family dwellings, please note the following information from Snohomish Health District's policy:

To determine the total daily flow for single-family homes with:

  • 1 to 3 bedrooms, use 150 gallons per bedroom per day
  • More than 3 bedrooms, use 120 gallons per bedroom per day


To review the entire Interpretation of Bedroom and Daily Flow Determination Policy (PDF)

Application Rates

Application rates shall be determined according to soil texture type as outlined in WAC 246-272A and the Environmental Protection Agency manual.

Minimum Land Area Requirements

The Minimum Land Area Requirement (WAC 246-272A) requires that minimum gross land area exists for a unit volume of sewage (450 gallons per day). Therefore, an on-site sewage disposal system application to create a new lot shall designate an area for at least a three-bedroom single-family dwelling. Note: This regulation has been in effect since July 1984.


Potential bedrooms are considered bedrooms regardless of the placement of closets, windows, and doors, or the width or location within the structure. 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 Health District typically does not count rooms such as living and dining rooms, storage areas, closets, and utility rooms as potential bedrooms.