The CII Permit 5-acre threshold does not use one calculation for every commercial, industrial, or institutional property. The correct measurement depends first on whether the site is covered by another NPDES stormwater permit. One facility may need to measure impervious surfaces, while another must evaluate its total area.
A property must also fall within the permit’s defined CII and geographic scope. Therefore, a five-acre parcel located somewhere in Los Angeles County does not automatically require coverage. A useful initial screen begins with the property’s permit status, then applies the correct acreage test and verifies the site’s classification, ownership, parcel configuration, and watershed location.
TL;DR
Which acreage threshold applies?
- The Los Angeles Regional Water Quality Control Board adopted Order R4-2026-0226 on July 23, 2026. The permit becomes effective October 31, 2026.1
- A qualifying CII site not covered by another NPDES stormwater permit is evaluated using 5 or more acres of impervious surface.
- A qualifying CII site covered by another NPDES stormwater permit is evaluated using 5 or more acres of total area.
- Five acres alone does not establish CII Permit coverage. The review must also confirm private ownership, CII classification, watershed location, existing permit status, and any applicable permit exception.
What does the Region 4 CII Permit’s 5-acre threshold actually mean?
Order R4-2026-0226, also identified as NPDES Permit CAS004005, creates two primary acreage tests for certain privately owned CII sites. Section 3.1 of the adopted permit applies the 5 acres of impervious surface CII Permit test to a qualifying site that is not covered by another NPDES stormwater permit. For a qualifying CII site covered by another NPDES stormwater permit, it applies the 5 acres of total area CII Permit test.1
The Los Angeles Regional Water Quality Control Board adopted the permit on July 23, 2026, and established October 31, 2026, as its effective date. Before that effective date, the permit should be described as adopted but not yet effective.1
The acreage threshold operates within a broader applicability framework. A potentially covered property must qualify as a privately owned CII site under the permit and must be located within one of the two specified watershed areas. Residential facilities are excluded from coverage under this General Permit.
For that reason, none of the following facts establishes coverage by itself:
- The parcel contains five or more acres.
- The property includes extensive roofing or pavement.
- A commercial or industrial business operates at the address.
- The address is within Los Angeles County.
- The facility has an IGP, NEC, NONA, or another stormwater permit status.
These factors must be reviewed together using the correct permit-status branch.
When does the permit use impervious surface instead of total area?
Order R4-2026-0226, also identified as NPDES Permit CAS004005, creates two primary acreage tests for certain privately owned CII sites. Section 3.1 of the adopted permit applies the 5 acres of impervious surface CII Permit test to a qualifying site that is not covered by another NPDES stormwater permit. For a qualifying CII site covered by another NPDES stormwater permit, it applies the 5 acres of total area CII Permit test.1
The Los Angeles Regional Water Quality Control Board adopted the permit on July 23, 2026, and established October 31, 2026, as its effective date. Before that effective date, the permit should be described as adopted but not yet effective.1
The acreage threshold operates within a broader applicability framework. A potentially covered property must qualify as a privately owned CII site under the permit and must be located within one of the two specified watershed areas. Residential facilities are excluded from coverage under this General Permit.
For that reason, none of the following facts establishes coverage by itself:
- The parcel contains five or more acres.
- The property includes extensive roofing or pavement.
- A commercial or industrial business operates at the address.
- The address is within Los Angeles County.
- The facility has an IGP, NEC, NONA, or another stormwater permit status.
These factors must be reviewed together using the correct permit-status branch.
When does the permit use impervious surface instead of total area?
The first acreage question is not “How large is the property?” It is “Is the CII site covered by another NPDES stormwater permit?”
Five acres equals 217,800 square feet, but the key compliance question is which area the permit requires the facility to measure.
If the qualifying CII site is not covered by another NPDES stormwater permit, Section 3.1.2 applies the threshold at five or more acres of impervious surface. In this branch, the property’s total acreage may be larger than five acres without satisfying the acreage test if less than five acres qualify as impervious surface.
For example, consider an unpermitted shopping or warehouse property occupying an eight-acre parcel. If its buildings, parking areas, roads, storage areas, and other qualifying impervious surfaces total only four acres, the parcel does not satisfy the five-impervious-acre test based solely on its eight-acre total size. The measurements and other applicability factors would still need verification before reaching a final conclusion.
If the qualifying CII site is covered by another NPDES stormwater permit, Section 3.1.3 instead uses five or more acres of total area. Impervious acreage should not replace total area in this branch.1
For example, an industrial facility with existing NPDES stormwater coverage may occupy six acres in total while containing fewer than five impervious acres. The five-acre total-area applicability test may still be relevant because the site is covered by another NPDES stormwater permit. The existing permit boundary must then be compared with the facility area to determine how the CII Permit applies to acreage addressed by the permit’s NEC, NONA, or partial-coverage provisions, as applicable.
Facilities comparing an existing Industrial General Permit and SWPPP program with the new CII requirements can review Industrial SWPPP Services.
What counts as impervious surface under the CII Permit?
Attachment A defines an impervious surface as one that prevents or significantly impedes precipitation from infiltrating into the underlying soil or groundwater. Examples include rooftops, driveways, sidewalks, walkways, patios, storage areas, roads, parking lots, asphalt, concrete, and certain compacted surfaces.1
A surface should not be classified only by its material name. Classification depends on whether the surface prevents or significantly impedes infiltration, including the permit’s specific treatment of compacted and infiltration-designed surfaces. For example, the permit identifies compacted gravel roads that inhibit infiltration as impervious. However, gravel and similar stabilized surfaces are not considered impervious unless they have been compacted specifically to prevent infiltration.
The permit also excludes qualifying surfaces designed to allow infiltration, including landscaped soil and certain pervious pavement systems. Interlocking pavers with permeable joints may qualify when installed over pervious soil or engineered storage media that permits infiltration. Railroad track ballast over pervious soil or appropriate storage media is another stated example.
An initial measurement may use:
- Current parcel and assessor information
- A boundary or site survey
- Existing civil or drainage plans
- Building and pavement measurements
- Geographic information system data
- Recent aerial imagery
- Field verification of surface conditions
Maps and aerial photographs can support a preliminary screen, but they may not reveal whether gravel is compacted, pavement is permeable, parcels operate as one site, or existing permit coverage applies to only part of the property. A final acreage position should rely on documented site and parcel information rather than a casual map measurement.
How do existing IGP, NEC, NONA, and partial-coverage sites affect the 5-acre test?
Existing stormwater documentation does not create an automatic exemption from the new CII Permit. The adopted order expressly addresses facilities with other NPDES stormwater coverage, including specific IGP-related situations.1
These existing permit situations can affect which acreage is evaluated and how existing permit boundaries interact with CII coverage. Facilities should therefore verify their current permit records, including the acreage and portions of the property associated with an NEC, NONA, or other NPDES coverage, before reaching an applicability conclusion. The Industrial Stormwater Permitting Process provides additional context for IGP, NEC, NONA, and permit-registration status.
This guide addresses these conditions only as part of the five-acre decision. Each status may require a more detailed document-level review.
Do contiguous parcels or campuses get counted together?
The permit does not always treat the assessor parcel containing the primary building as the entire site. Its definition of a CII site includes a privately owned parcel or contiguous parcels classified as commercial, industrial, or institutional under the specified Los Angeles County Assessor land-use codes.1
The permit also provides for certain multi-parcel campus and consolidated-coverage arrangements. As a result, an applicability screen should not automatically stop at one APN when related parcels are under common ownership or operation or form part of a larger facility configuration.
The review may need to identify which parcels form the relevant CII site, how existing NPDES coverage applies across them, and whether facility operations or drainage cross parcel boundaries. These provisions do not mean that every nearby parcel must automatically be combined; complex multi-parcel properties require a documented site and parcel review.
Does a 5-acre property automatically need CII Permit coverage?
No. Passing one of the acreage tests is only part of the CII Permit coverage analysis.
The property must be a privately owned CII site within the permit’s defined land-use scope. The permit uses specified Los Angeles County Assessor land-use codes to define commercial, industrial, and institutional sites and excludes residential facilities from coverage.1
Mixed-use parcels require a more specific review. Attachment A establishes separate treatment for land-use codes 1210 and 1720, including exclusions for residential building footprints and a 50% or more non-residential building-area test for certain common areas and shared parking. A mixed-use parcel should therefore not be screened by assigning its entire acreage to CII use. 1
The property must also be located within:
- The Dominguez Channel/Los Angeles and Long Beach Inner Harbor Watershed; or
- The Los Cerritos Channel/Alamitos Bay Watershed.
EPA’s residual designation applies exclusively to qualifying privately owned CII sites within these two Los Angeles County watersheds.2 A property does not fall under this specific Region 4 CII Permit merely because it is located elsewhere in Los Angeles County or elsewhere within Region 4.
Existing individual permit requirements create another necessary gate. Section 3.1.4 states that a facility with an individual NPDES permit containing requirements at least as stringent as those in the CII Permit is not required to obtain coverage under this General Permit.1
That exception should be supported by reviewing the actual individual permit. The presence of an individual NPDES permit does not, without comparison of its requirements, establish that the exception applies.
How to Run a Defensible 5-Acre CII Applicability Screen
As a practical screening framework, an initial review can document the information used to select the acreage test and evaluate the permit’s other applicability criteria.
- Confirm the address, APN, and CII land use. Identify the property boundary, ownership, facility use, and applicable Los Angeles County Assessor land-use classification.
- Verify the watershed. Determine whether the property falls within the Dominguez Channel/Los Angeles and Long Beach Inner Harbor Watershed or the Los Cerritos Channel/Alamitos Bay Watershed.
- Review current permit status. Identify any NPDES stormwater permit, Industrial General Permit coverage, NEC, NONA, or other permit affecting the site.
- Select the correct acreage test. Use impervious surface for a qualifying CII site not covered by another NPDES stormwater permit. Use total area for a qualifying site covered by another NPDES stormwater permit.
- Measure and document the relevant area. Use reliable parcel, plan, survey, GIS, and field information. Record the assumptions used to classify surfaces.
- Identify edge cases. Review partial permit coverage, contiguous parcels, campus configurations, NEC or NONA acreage, and any individual NPDES permit that may contain sufficiently stringent requirements.
- Obtain technical confirmation where the record is unclear. A screening can identify likely applicability and the next compliance step, but it does not guarantee the Water Board’s final regulatory determination.
If the screen indicates that coverage is required, the next step is to determine the applicable enrollment and compliance schedule for the facility, including the NOI, site-specific SWPPP, initial sampling results, and Compliance Option Documents required by the permit. For related permitting assistance, see SMARTS Permitting and Application Services.
SoCal Stormwater Solutions can conduct a Region 4 CII applicability review using the property’s actual parcel and permit records. To request a review, provide the site address and APN, current NPDES or IGP status, any NEC or NONA documentation, total acreage, estimated impervious acreage, and known watershed or drainage information to request a technical compliance review.
