New Construction Stormwater General Permit Released: Effective January 1, 2026

Washington State Department of Ecology has released the updated 2026 Construction Stormwater General Permit (CSWGP), which will go into effect on January 1, 2026. This permit governs stormwater discharges associated with construction activity across the state and includes several important updates that builders, developers, municipalities, and contractors will need to understand ahead of the new year.

Below, you’ll find links to the permit itself, Ecology’s official fact sheet and response to comments, and a summary of the key changes.

The 2026 Construction Stormwater General Permit

The full permit is now available and will go into effect starting January 1, 2026.

Read the full permit here:

2026 Construction Stormwater General Permit

Construction Stormwater General Permit Fact Sheet with Response to Comments

Ecology also released an accompanying Fact Sheet and Response to Comments, which provides:

• The technical and regulatory basis for permit provisions

• Ecology’s rationale behind new or revised requirements

• Responses to public comments submitted during the permit update process

Read the fact sheet with response to comments here:

Fact sheet for the draft 2026 general permit with Response to Comments

What is New in the 2025 Permit

Changes to S1: Permit Coverage

Clarification of what triggers the minimum requirements for
coverage

The 2026 permit provides more explicit and expanded language describing what qualifies as construction activity requiring coverage.
Notable clarifications include:

• A more detailed list of soil-disturbing activities, including clearing, grading, excavation, construction support activities, haul roads, borrow areas, side-cast areas, portable rock crushers, staging yards, and other on- or off-site support areas.

• Construction Support Activity is now embedded directly into the coverage section with clearer examples, making it obvious when support yards must be covered under the same permit.

Updated prohibited discharge language about wheel washes to match special conditions in S9

The 2026 permit updates the prohibited discharge language to match the wheel-wash provisions in S9:

• Discharge of wheel wash wastewater is prohibited unless it goes to a closed-loop recirculation system or is applied upland, aligning S1 and S9 to avoid conflicting interpretations.

Updated S1.E.3- federal operator to match recent changes from EPA

The 2026 permit updates the text describing where Ecology’s permit authority does not apply:

• Clarifies that the permit does not cover stormwater discharges from any federal operator or Lands of Exclusive Federal Jurisdiction, aligning the state permit with recent EPA terminology updates.

Changes to S2: Application Requirements

Improved understanding of what constitutes contamination, and what information to provide with the Notice of Intent (NOI) about the contamination

The 2026 permit adds additional detail to the NOI requirements regarding contaminated soil or groundwater:

• Expands “contamination” to include known, remediated, or historically contaminated soil or groundwater.

• Now requires a “brief project overview” as part of the NOIs detailed supporting information

Modifications that increase acreage and partial transfers will need to provide updated site maps with their form.

The 2026 permit adds a new requirement:

• When a permittee submits a modification of coverage for updating their permitted acreage or partial transfer, the submittal must include an updated site map showing the revised permitted acreage.

Removed S3.A- based on 2025 Supreme Court decision (San Francisco vs. EPA)

The updated permit removes the old S3.A language regarding compliance with water quality standards.
This removal reflects the 2025 Supreme Court ruling (City of San Francisco v. EPA), which limited the use of “end result” narrative based permit requirements in NPDES permits.

S4. Monitoring Requirements, Benchmarks, and Reporting Triggers

All site inspections will be conducted by a CESCL, regardless of disturbed acreage.

In 2020, sub-acre sites did not require CESCL inspections.
In 2026:

• Every site, regardless of size, must use a CESCL for weekly and rainfall-triggered inspections.

All sites under 5 acres will be required to conduct weekly turbidity and pH sampling.

The 2026 permit now requires:

• All sites must conduct weekly turbidity sampling, and if significant concrete work/engineered soils are present, pH sampling must also be performed. (previously sites under an acre did not require sampling)

• This replaces some ambiguity in the 2020 permit regarding when pH sampling is triggered.

Clarification about when DMR submittals are required.

The 2026 permit clarifies:

• Monthly DMRs are required for the entire duration of permit coverage, regardless of whether the site discharged that month.

• This was implied in 2020 but is explained more clearly in the updated permit.

Only calibrated pH meters are allowed for pH testing.

2026 strengthens pH requirements:

• pH testing must use a calibrated pH meter, removing ambiguity about using pH strips or other methods.

• Calibration procedures must follow manufacturer instructions.

Added language about CESCL inspections about looking for evidence of discharges even when there isn’t active flow.

The 2026 permit adds new language requiring CESCLs to:

• Inspect for evidence of past discharges, even if water is not actively discharging during the inspection.

• This includes staining, erosion, flow marks, or sediment deposits.

• CESCL should also examine stormwater visually at all stormwater discharge points

Required calibrations of pH meters to be maintained in the logbook.

• pH calibration records now need to be maintained within the logbook kept on site

Clarified language about sampling locations

• Clarifies the language outlining where sampling needs to occur
  • Onside discharges to surface waters
  • Inlets, open conveyance, or any offsite discharges including those to ground

S5. Reporting and Recordkeeping

Changed language about when ERTS should be reported

• Permittees must report turbidity of 250 NTUs or more to ecology within 24 hours of becoming aware of a failure to comply.
• Permittees must report any noncompliance to ecology within 24 hours of the failure to comply

S8. Discharges to 303(d) or TMDL Waterbodies

Including dissolved oxygen to the list of limit triggers for
impaired waterbodies.

The 2026 permit expands S8 to include:

• Low dissolved oxygen as an impairment triggering numeric effluent limits for turbidity.

Updated sampling frequency requirements for site that are
dewatering.

The 2026 permit now requires:

• Daily turbidity sampling of dewatering water if discharged to water impaired for turbidity, fine sediment, phosphorus.

S9. Stormwater Pollution Prevention Plan

Updated Establish Construction Access, Protect Slopes,
Stabilize Channels and Outlets, Protect Infiltration BMPs

Construction Access:

• New permit prohibits the use of crushed concrete for construction access stabilization
• Reiterates that wheel washes and tire baths must comply with special condition S9.D.9.d

Protect Slopes:

• Clarified the methods used to size pipe slope drains
• Single Hydrograph method OR
• Continuous simulation method- which now uses a peak flow rate determined by an approved continuous runoff model with a 15-minute time step

Stabilize Channels and Outlets

• Clarifies the methods used to size conveyance channels as mentioned in the protect slopes section
• Clarifies that construction road shoulders and sloped work areas should be stabilized in addition to outlets, adjacent stream banks as previously required

Protect Infiltration BMPs

• Adjustments to language surrounding LID and protection of Infiltration BMPs broadening what needs to be protected to include all existing and proposed infiltration BMPs

Clarified that potentially contaminated containment water is not to be discharged

• New permit language stating that containment water that is free from indicators of contamination may be discharged with other stormwater, but visibly contaminated water can’t be discharged and should be disposed of appropriately.

Added language about BMPs related to demolition debris.

• Added requirement that demolition of any buildins built before 1980 must implement BMP S438 of the SWMM
• These of BMPs largely target the prevention of PCB containing building materials from entering your stormwater discharge

Added/Updated Definitions

Construction Activity definition broadened to include “other soil disturbing activities”

Construction Support Activity: Provides clarification and examples as to what constitutes construction support activity,

• includes on or off site areas
• Includes but are not limited to : equipment staging, burrow pit, material storage areas, dump areas, haul roads, construction roads, side-cast areas, on-site portable rock crusher, staging yards, parking areas, off-site construction support activities and all other soil disturbing activities.

Date of Receipt – This is defined in RCW 43.21B.001(2) as five business days after the date of mailing; or the date of actual receipt, when the actual receipt date can be proven by a preponderance of the evidence. The recipient’s sworn affidavit or declaration indicating the date of receipt, which is unchallenged by the agency, constitutes sufficient evidence of actual receipt. The date of actual receipt, however, may not exceed forty-five days from the date of mailing.

Days (compliance period interval) – When the compliance period is stated in days: (A) exclude the day of the event that triggers the period; (B) count every day, including intermediate Saturdays, Sundays, and legal holidays; and (C) include the last day of the period, but if the last day is a Saturday, Sunday, or legal holiday, the period continues to run until the end of the next day that is not a Saturday, Sunday, or legal holiday.

Groundwater Discharge Point (or Discharge to Groundwater) means the location where stormwater associated with industrial activity enters a stormwater infiltration structure that is used, intended or designed to infiltrate water into the ground

Operational Hours means when work is happening on site related to the project and project support activities, whether the activities are scheduled or unscheduled.