Post-Construction Stormwater and LID in California: What the Draft Small MS4 Permit Means for New Development

If your project adds or replaces roofs, paving or other hard surfaces, the city or county will ask how you will manage stormwater after construction. The statewide rules for smaller cities and counties are now under review. On July 2, 2026, the State Water Resources Control Board released the Draft Small MS4 Permit (NPDES No. CAS000004), a proposed replacement for the statewide Phase II permit adopted in 2013.

What are post-construction stormwater requirements in California?

They govern the permanent drainage features a project leaves behind, not the temporary erosion controls used during construction. They come from municipal stormwater permits. An MS4 (municipal separate storm sewer system) is the network of streets, inlets and pipes that carries runoff to creeks and the ocean, and each MS4 permit requires the city or county to make new development control its runoff.

Your city or county can tell you which permit applies.

The design standard is Low Impact Development (LID): letting rain soak in, evaporate, be reused or filter through soil and plants near where it falls. On a site plan, that means:

• Bioretention planters
• Vegetated swales
• Infiltration trenches
• Pervious pavement
• Green roofs
• Cisterns

What is the Small MS4 permit, and what is changing in the 2026 draft?

The current permit was adopted February 5, 2013, and has been administratively extended since February 5, 2018. After the draft's release and a revised notice on July 8, staff held workshops July 29 and 30, the Board held a hearing August 19 with no action taken, and written comments closed at noon on September 2, 2026.

Everything in the draft is proposed, not adopted. Its fact sheet says the project-size triggers and general flow of the LID requirements are unchanged. The proposed changes that matter most for development:

• Large single-family homes. The 2013 permit excludes detached single-family homes that are not part of a larger plan of development. The draft removes that exclusion, so a home creating 5,000 square feet or more of impervious surface would become a Regulated Project.
• Redevelopment. The 50 percent rule would apply to altering existing impervious area, not only to increasing it.
• Road work. Full-depth reclamation that does not change drainage patterns is spelled out as excluded maintenance.
• Site design menu. Soil quality improvement is dropped because Regional Board inspectors found it hard to verify.
• Offsite options. Agencies could let a project meet all or part of its retention or peak-flow requirement offsite, normally in the same watershed or through an approved watershed or regional plan, with offset projects finished within three years of the project's completion unless the Regional Water Board's Executive Officer allows longer.

Does my project have to meet LID requirements if it adds 5,000 square feet of impervious surface?

In Phase II areas, usually yes. Under the current permit and the draft, a Regulated Project creates or replaces 5,000 square feet or more of impervious surface (the draft counts this across the entire project site) and falls under the agency's planning or building authority. It must meet the full LID standards and submit a stormwater control plan.

Projects that create or replace 2,500 to under 5,000 square feet, and are not part of a larger plan of development, are what the draft calls Small Projects. They need applicable source controls, such as covered trash areas, plus at least one site design measure, like routing downspouts to landscaping, documented on a checklist.

Redevelopment has its own test. Under the draft, if a project alters or increases a site's existing impervious surface by 50 percent or more, controls must treat runoff from all existing, new and replaced impervious surfaces of the project.

The current permit applies this rule only to increases of more than 50 percent.

Interior remodels and routine maintenance are excluded. The current permit's examples are exterior wall surface replacement and pavement resurfacing within the existing footprint; the draft adds roof replacement and limits the repaving exclusion to work that does not expose the soil below. Under the draft, public frontage improvements built with a private project are included in its LID design.

How are LID treatment and retention facilities sized?

A Regulated Project must meet at least one of these sizing criteria:

• Volume-based: capture the runoff from approximately the 85th percentile 24-hour storm (per WEF Manual of Practice No. 23 / ASCE Manual of Practice No. 87), or at least 80 percent of annual runoff using CASQA's handbook method (the 2003 New Development and Redevelopment handbook under the current permit; the most recent Development BMP Handbook, June 2021, under the draft) and local rainfall data.
• Flow-based: treat the runoff from a rainfall intensity of at least 0.2 inches per hour, or twice the local 85th percentile hourly intensity.

Facilities must perform at least as well as baseline bioretention: a maximum surface loading rate of 5 inches per hour, at least 6 inches of surface ponding, at least 18 inches of planting media that infiltrates at least 5 inches per hour, and at least a 12-inch gravel layer.

The permit allows a sizing factor of 4 percent of the tributary impervious area (the 0.2-inch-per-hour design intensity divided by the 5-inch-per-hour loading rate), so 10,000 square feet of roof and paving needs about 400 square feet of bioretention surface.

Baseline bioretention, in section

Illustrative diagram — not to scale

1. 1Roof and paving: the tributary impervious area that drains to the facility. The permit allows a sizing factor of 4 percent of that area.
2. 2Surface ponding: at least 6 inches.
3. 3Planting media: at least 18 inches, infiltrating at least 5 inches per hour.
4. 4Gravel layer: at least 12 inches.

When are hydromodification or peak-flow controls required?

Hydromodification is the change development makes to how water moves across and through a site, such as more runoff volume and higher, longer peak flows, which can erode downstream creek banks and channel beds.

Regulated Projects that create or replace one acre or more of impervious surface must keep post-project peak flows at or below pre-project peaks for a storm set by geomorphic province:

• 2-year, 24-hour storm in provinces including the Coast Ranges, Klamath Mountains, Cascade Range, Modoc Plateau, Basin and Range and Sierra Nevada.
• 10-year, 24-hour storm in the Transverse Ranges, Peninsular Ranges, Mojave Desert and Colorado Desert.

Projects that do not increase impervious area are exempt from peak-flow control. The draft would also exempt projects in catchments that drain entirely through concrete-lined channels or enclosed pipes running continuously to a bay, delta, ocean or flow-controlled reservoir. One facility can be designed to meet both requirements.

What goes into a stormwater control plan?

The draft requires a Post-Construction Stormwater Control Plan for each Regulated Project, and the agency may not approve construction of impervious surfaces until the plan shows compliance.

It includes:

• New and replaced impervious area.
• A site assessment summary.
• A map of Drainage Management Areas (DMAs).
• The controls for each DMA.
• Sizing calculations.

The owner must also accept long-term maintenance through a signed statement, deed language, CC&Rs or a similar agreement, and the agency verifies the facilities before occupancy.

How do tributary area, pipe sizing and basin calculations fit together?

They build on each other. Each DMA is the tributary area draining to one point or facility; its measured roof, paving and landscape set the water-quality volume or flow that sizes each LID facility.

Storm drain pipes and inlets are then sized for the design storm in the local drainage manual, and any detention basin is sized to the agency's detention criteria; for projects that trigger hydromodification control, that means holding post-project peaks at or below pre-project peaks for the required storm. Requirements vary by county.

When would the new Small MS4 permit take effect?

Not yet. The draft says the order becomes effective six months after adoption, and no adoption date has been announced. Each city and county would then have up to two years to adopt or update its LID criteria and codes.

A project's status then matters: the draft applies new requirements to, for example, discretionary projects not yet deemed complete for processing.

Do these rules apply everywhere in California?

• Phase I areas. Cities and counties under a Phase I permit, including small cities that are co-permittees on a countywide permit, follow their Regional Water Board's municipal permit, such as Order R2-2022-0018 in the San Francisco Bay region or Order R4-2021-0105 in the Los Angeles region, with their own thresholds and sizing methods.
• Central Coast. Phase II agencies there follow requirements the Central Coast Water Board adopted in Resolution R3-2013-0032 on July 12, 2013, using a watershed-process approach. The draft lets them continue.
• Outside any MS4. Construction sites covered by the 2022 Construction General Permit (one acre or more of disturbance, or part of a larger common plan of development) that lie outside an NPDES-permitted MS4 must meet that permit's post-construction requirements, uploading plans and calculations in SMARTS with their permit registration documents. Linear underground and overhead projects are exempt from those requirements.

Inside an MS4, the local agency's adopted standards control; outside one, the Construction General Permit's post-construction requirements apply.

What this means for your project

1. Ask early. Ask planning or public works which post-construction program applies, and request its LID manual, checklist and sizing tool.
2. Count impervious area carefully. Tally created and replaced area, including frontage work, and test the 50 percent redevelopment rule.
3. Give LID room on the site plan. At a 4 percent sizing factor, bioretention takes real ground. Placing it during civil site layout avoids losing parking or building area later.
4. Coordinate grading and drainage. Bioretention depths, overflow elevations and storm drain connections belong in the grading plans and street and storm drain improvement plans from the start.
5. Watch the final permit. If your application may land after local standards change, check the draft's narrower exemptions now, especially for large custom homes.

We prepare stormwater calculations, from LID sizing to pipe and basin calculations, as part of a coordinated plan set. See our stormwater and erosion control services, or contact us about your project with your site plan and agency comments.