March 10th, 2025

OSS DESIGNERS,

This special edition newsletter is being sent to all OSS designers, and below we have highlighted a couple more upcoming WAC revisions relevant to you. This notification is not all-inclusive of the revisions to WAC 246-272A effective April 1, 2025 but is intended to highlight some of the key changes to the rule. 

Reminders

• The complete Chapter 246-272A WAC, as well as a Summary of Rule Changes, Significant Analysis, and other supporting documents can be found by visiting the Washington State Department of Health on-site sewage system rule revision webpage.
• Renewal septic applications submitted April 1st, 2025 and after will need to meet new code requirements.

WAC 246-272A-0280 

Highlights: Repair of Failures

• Clarification that the LHO may permit OSS that meet Table X only in cases of repairs. 
• Clarified owner’s options in cases of failure. 
• Added new Table IX Options and Methods to Address an OSS Failure to explain owner’s options in case of OSS failure. 
• The OSS owner shall notify the local health officer when there is a failure and indicate which methods will be used to address the failure in accordance with Table IX. 
• Added requirement that OSS designer evaluate the causes of failure prior to designing the repair or replacement of an OSS. 
• Updated and renamed Table X, Treatment Component Performance Levels for Repair of OSS Not Meeting Vertical and Horizontal Separations, to incorporate changes made to treatment standards (separating Bacterial Levels from other treatment levels) and to correct inconsistencies with WAC 246-272A-0230. 
• Click here to view Table IX and Table X

WAC 246-272A-0320 

Highlights: Developments, Subdivisions, and Minimum Land Area Requirements

• Changed title of Table XI 
• Increased minimum land area requirement in Table XI, Minimum Land Area Requirement for Each Single-Family Residence or Unit Volume of Sewage, for lots served by a public water source, by 500-1,000 square feet, depending on soil type. This changes the minimum lot size on Table XI from 12,500 to 13,000 square feet for all lots served by public water supplies. 
• Added “Minimum Usable Land Area” requirement to Table XI. This is a new requirement defined in WAC 246-272A-0010 to mean the minimum land area within the minimum lot size required per development using an OSS, which is based on soil type and type of water supply. Minimum usable land area is free of all physical restrictions and meet minimum vertical and horizontal separations. 
• Removed references to Method I and Method II lot sizing methods. 
• Added new methodology and new Table XII, Maximum Allowable Total Nitrogen (TN) Load Per Day by Type of Water Supply, Soil Type, and Land Area for developments that do not meet Table XI’s requirement. The Washington State Department of Health (DOH) is expected to release technical guidance for applying WAC 246-272A-0320, including Table XI and Table XII.
• Changed minimum lot size from 12,500 to 13,000 sq. ft. for nonpublic water supplies for all new lots. 
• Reduced the maximum unit volume of sewage per day per acre from 3.5 to 3.35 for lots served by public water supplies for both Table XI and Table XII due to the change of minimum lot size from 12,500 to 13,000 square feet for all lots served by public water supplies on Table XI. 
• Added clarifying language that LHOs may allow permitting of an OSS on a preexisting lot of record that does not meet current minimum land area requirements only if it meets all requirements of chapter 246-272A WAC without the use of a waiver under WAC 246-272A-0420.
• Click here to view Table XI and Table XII

Questions? Contact us!

Email: SHD-EHQ@snoco.org

Call: (425) 339-5250