C.3 LID Requirements: What Qualifies as Low Impact Development?

C.3 is the stormwater provision embedded in the Municipal Regional Permit (MRP) that governs new development and redevelopment across most of the San Francisco Bay Area. It requires projects that create or replace a certain amount of impervious surface to manage stormwater on-site using Low Impact Development (LID) techniques — meaning you infiltrate, evapotranspirate, or harvest and reuse runoff rather than routing it straight to the storm drain. The short answer: if your project disturbs enough impervious area, you must demonstrate that post-construction stormwater is treated, and in many cases retained, using compliant LID measures sized to a specific design storm.

LID under C.3 is not a stylistic preference. It is a permit condition tied to the MRP issued by the San Francisco Bay Regional Water Quality Control Board (RWQCB) under the Clean Water Act’s NPDES program. Non-compliance can stall your project at building permit issuance, trigger enforcement from your local Permittee agency, or create problems at close-out when your Qualified SWPPP Practitioner (QSP) conducts the final inspection. Understanding exactly what triggers C.3 — and what doesn’t — saves significant time and money during entitlement and design.

When Does C.3 Apply — and When Does It Not?

C.3 applies to projects within the 76 Permittee jurisdictions covered by MRP Order R2-2022-0018 (the current fourth-term MRP). The two primary triggers are:

• New development: Projects that create 10,000 square feet or more of impervious surface.
• Redevelopment: Projects that replace 10,000 square feet or more of impervious surface, or that replace 50% or more of the impervious surface on a site where the existing impervious area already exceeds 10,000 square feet.

If your project falls below the 10,000-square-foot threshold, C.3 LID requirements generally do not apply — though your local Permittee may have more stringent local standards, so always check. Projects between 2,500 and 10,000 square feet of impervious surface typically trigger C.3 Basic Requirements (site design measures and source controls) but not the full Regulated Project LID treatment requirements. Only projects at or above the 10,000-square-foot threshold must size and document LID treatment measures.

There are also categorical exemptions. Routine maintenance and repair, interior remodels, emergency projects ordered by a public agency, and projects that disturb only landscaping typically fall outside C.3’s scope. Linear underground utilities are another nuanced case — they may disturb significant soil but create little net impervious surface, so they often avoid full C.3 obligations.

What Does C.3 LID Treatment Actually Require?

For Regulated Projects (≥10,000 sq ft impervious), C.3 requires on-site stormwater treatment sized to capture and treat the 80th percentile 24-hour storm event for the project location. That’s the volume threshold — you must retain or treat runoff from storms up to that depth. In most Bay Area jurisdictions, this works out to roughly 1 to 1.5 inches of rainfall depending on your location’s IDF data.

LID measures that satisfy this requirement include:

• Bioretention cells (rain gardens): Engineered landscape depressions that filter runoff through amended soil media, typically 18–24 inches deep, with an underdrain where infiltration rates are low.
• Infiltration trenches and basins: Acceptable where native soils have infiltration rates above 0.3 in/hr and seasonal high groundwater is more than 10 feet below the basin bottom.
• Permeable pavement: Structural pavement systems — permeable concrete, porous asphalt, or interlocking pavers — that allow water to pass through and either infiltrate or be detained in a sub-base reservoir.
• Green roofs: Used to reduce effective impervious area, though they require supplemental treatment at grade in most designs.
• Rainwater harvesting and reuse systems: Acceptable when sized to retain the design volume, and when a use pathway (irrigation, toilet flushing) can be documented to avoid overflow during wet seasons.

The MRP also requires projects to prioritize infiltration where feasible. If a geotechnical investigation (per your local agency’s requirements, which often reference ASTM D3385 or equivalent field percolation testing) shows native soils can support infiltration, you must use it. Flow-through planters and media filters that discharge to the storm drain — rather than infiltrate — are permissible only where infiltration is infeasible per documented criteria in the MRP Provision C.3.d.

How Does C.3 Interact With Other Permit Requirements?

C.3 does not exist in a vacuum. On projects that also disturb one acre or more of soil, the California Construction General Permit (CGP, Order 2022-0057-DWQ) requires a Stormwater Pollution Prevention Plan (SWPPP) prepared and implemented by a QSD. Post-construction LID measures documented under C.3 must be consistent with the long-term BMPs described in the SWPPP’s post-construction section.

If your project involves a subdivision, the Subdivision Map Act (Government Code §66426 et seq.) may require you to record a Stormwater Management Facilities Operation and Maintenance Agreement as a condition of final map approval. This is how your local agency ensures the bioretention cell you build today still functions in 25 years — maintenance obligations run with the land.

For projects subject to California Building Code (CBC) accessibility requirements under CBC Chapter 11B, LID features like bioretention cells and permeable pavement must be designed so they don’t create tripping hazards or interrupt accessible routes. A bioretention inlet flush with a walking surface is a different design problem than a curb cut with a grate — both must meet accessibility standards and LID function simultaneously.

High-priority projects near the Bay, wetlands, or sensitive receiving waters may also face additional requirements under the Bay Area’s Trash Load Reduction provisions (MRP Provision C.10) or special conditions tied to a 401 Water Quality Certification from the RWQCB.

A Practical Example: 10-Unit Condo on a 0.8-Acre Site in Oakland

Say you’re building a 10-unit residential condo project in Oakland on an 0.8-acre infill lot. The existing site is a surface parking lot — nearly 100% impervious. Your project proposes a 4-story building with a ground-floor garage, replacing virtually all of the existing impervious surface with new building footprint, driveway, and hardscape.

Because you’re creating and replacing well over 10,000 square feet of impervious surface, you’re a Regulated Project under C.3. Oakland falls under the Alameda Countywide Clean Water Program, your local Permittee, which follows the MRP and publishes its own C.3 Technical Guidance. You’ll need to calculate the design treatment volume for the 80th percentile storm, then size bioretention cells — likely integrated into your landscape planters along the building perimeter or in a rear courtyard — to handle that volume.

Infiltration testing (typically a Percolation Test per your geotech’s scope) will determine whether you can infiltrate or need underdrains discharging to the storm drain. Because this is infill Oakland with shallow utilities and compacted urban soils, underdrains are common. You’ll document the LID sizing in a Stormwater Control Plan (SCP), submit it with your building permit application, and record an O&M Agreement before the City signs off on the final map or issues a certificate of occupancy.

The soil amendments, overflow structure, underdrain cleanout locations, and planting palette all need to be shown on civil drawings. If you skip this during design development and catch it at permit submittal, you’re redesigning your grading plan and