Septic System Sizing

Septic System Sizing

As you go through the septic application and design process, the licensed septic designer or professional engineer you hire will determine the appropriate septic system size. The main factor they will look at to determine the size of your system is the number of bedrooms in the house. 

Why does the number of bedrooms matter?

The size of your septic system must be able to handle the amount of waste generated by your household. How much waste a household creates is generally dependent on the number of people living there. However, it is not just the number of people who currently live in the house, but the number of people that could potentially live in the house in the future.

For example, a couple wants to build a new, four-bedroom house. Their septic system must be sized as if all four of those bedrooms will be occupied even though only two people plan to live there. This is because another family could eventually move in and will need a system capable of handling more waste. This is why the number of bedrooms in your home impacts the size of your septic system.

What is considered a bedroom?

The Health Department considers potential bedrooms as 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:

  • Dens
  • Offices
  • Sewing, computer, music, exercise or hobby rooms
  • Studies
  • Lofts
  • Unfinished rooms
  • Bonus rooms
  • Libraries

Living rooms, dining rooms, storage areas, closets, and utility rooms as not typically considered potential bedrooms.

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

Septic system types

There are several different types of septic systems, some are basic and others are complex. In addition to determining the size of your septic system, the licensed professional designing your system will also determine what type of septic system is suitable for your property. They will consider multiple factors such as the type of soil on your property, how much space you have, depth to the water table, and how close you are to a water source (such as a well, river, or lake).

Your licensed professional will determine what is necessary for your property so that it meets Washington State regulations and Snohomish County Board of Health Code for onsite wastewater treatment.