New stormwater management requirements will splash down on roughly 600 sites in Los Angeles County starting October 31, 2026 under the new Commercial, Industrial, and Institutional (“CII”) permit.

The permit imposes on owners and operators of qualifying facilities extensive requirements, tight deadlines, high compliance costs, and stiff penalties.

Here are a few questions you should be asking now to prepare:

Is my site subject to permit requirements?

If your site meets all of the following criteria, then CII permit requirements likely apply:

• Location. Your site lies within one of two watersheds, as shown on the map provided by the Los Angeles Regional Water Quality Control Board. One covered watershed drains to the Dominguez Channel and the inner Los Angeles and Long Beach Harbor. The other drains to Los Cerritos Channel and Alamitos Bay.
• Use. Your parcel is privately owned, non-residential, and the Los Angeles County Assessor has assigned it a commercial, industrial, or institutional land use code.
• Size. Your site includes five or more acres of impervious surface, meaning pavement or other surfaces that do not absorb rain, such as parking lots, driveways, and rooftops.

Who is responsible for compliance?

Whether the owner or operator of a site is required to comply depends on who controls operations, but both can incur penalties for noncompliance.

What do I need to do and when?

Sites operating on October 31, 2026 face three deadlines:

• October 31, 2027, for the notice of intent and the pollution prevention plan.
• April 30, 2028, for the laboratory results from the initial sampling.
• April 30, 2030, for the compliance option documents.
• Annual reports are due by December 15 of each reporting year.

The phase-in period does not apply to sites that begin discharging after October 31, 2026. Those sites must file the required package, including the notice of intent, the pollution prevention plan, initial sampling results, and compliance option documents, at least 45 days before discharge begins.

The permit does not allow extensions. Other deadlines related to pollution discharge limits will go into effect over the next ten years, expanding requirements and potentially increasing costs.

How will I know which compliance option is best, and can I change my mind?

Compliance requires a pollution prevention plan tailored to each site. It also requires one of three options for meeting numeric limits on pollutants:

• Option 1 is a binding agreement to help fund a regional stormwater project already planned by the local watershed management group, which is the coalition of cities responsible for the municipal storm drain system.
• Option 2 is building and maintaining controls on the site itself, sized to capture, soak in, reuse, or divert a minimum quantity of rainwater.
• Option 3 is proving compliance by sampling the runoff and reporting the results. It is the only option under which an exceedance is measured rather than presumed away.

Changing compliance options 1 or 3 requires 30 days written notice, a justification, and replacement documents. Option 2 enrollment commits the site to this option for at least two years.

How much is this going to cost?

The CII Permit imposes significant compliance costs. The Board estimated each option as an annual cost for each acre of impervious surface (in 2023 dollars):

• Option 1, about $4,100 per acre.
• Option 2, about $21,655 to $24,694 per acre, including an up-front capital outlay of $285,000 to $325,000 for each acre-inch of storage. This estimate does not include any necessary pretreatment equipment.
• Option 3, anywhere from $660 to $30,666 per acre. The low figure assumes the site already meets the limits and pays only for sampling. These estimates assume the site must build Option 2 controls and must sample.

Other compliance costs will apply, including sampling costs of roughly $1,535 per storm event.

What happens if I don’t comply?

The cost of noncompliance can be stiff. California law requires a minimum $3,000 penalty for each serious violation. Failing to file a required monitoring report is treated as a serious violation. Administrative and court penalties, criminal liability, and required corrective action could add to these costs.

And the risks do not end there. All reports and plans must be submitted electronically through the state’s stormwater tracking online database. The Clean Water Act allows a citizen suit to enforce permit compliance, which is often based on those filings — or based on a site’s failure to file. In a citizen suit, a successful plaintiff may recover attorneys’ fees.

If you have any questions about whether or how these updates may affect you, please contact Hanson Bridgett’s Environmental and Water lawyers.