You're in a pre-app meeting with the city, or maybe your architect just handed you a comment letter, and somewhere in the stack of requirements you see the words "C.3 compliance." Nobody stops to explain it. Everyone else in the room nods like they know exactly what it means. You nod too.
So what's C.3 stormwater, really?
Here's the short version: C.3 is a provision in the Municipal Regional Stormwater Permit (MRP) that requires new and redevelopment projects in the San Francisco Bay Area to manage stormwater runoff on their own site. You can't just pave everything and let the rain drain to the street anymore. You have to capture it, treat it, and slow it down before it leaves your property.
The longer version is what this post is for.
Why Does C.3 Even Exist?
Think about what happens when it rains on a parking lot. The water picks up oil, tire rubber, heavy metals, and fertilizer runoff, and it carries all of that into the storm drain — which, in most Bay Area cities, flows directly to the nearest creek or bay. No treatment. No filtration. Straight in.
The federal Clean Water Act has required cities to address this since the early 1990s. The mechanism is called an MS4 permit — a Municipal Separate Storm Sewer System permit. In the Bay Area, that permit is issued by the San Francisco Bay Regional Water Quality Control Board, and it governs 76 cities and counties across nine counties. The current version is called the Municipal Regional Stormwater Permit, or MRP.
Provision C.3 of that permit is the piece that reaches down to individual development projects. It says: if you're building something that creates or replaces enough impervious surface, you're responsible for treating and controlling the stormwater that falls on your project.
This isn't about flooding. It's about water quality. The Bay is a protected water body. The creeks feeding it are regulated. C.3 is the mechanism that puts the burden of pollution prevention on the projects generating the runoff — which is exactly where it belongs.
What Triggers C.3?
This is where developers usually start paying attention. The thresholds are area-based, and they're measured in square feet of impervious surface created or replaced.
The standard C.3 trigger under MRP 3.0 is 5,000 square feet of new or replaced impervious surface for most project types. Single-family homes remain at 10,000 SF; C.3.i small-project requirements apply at 2,500 SF. That's not a huge parking lot. A two-story office building with a surface lot can hit that easily. So can a gas station rebuild, a grocery store pad, or a multi-family project on a tight infill site.
Some municipalities have lower thresholds than others — a handful of jurisdictions in the Bay Area have adopted stricter local standards. Some project types (like single-family homes on lots under a certain size) are conditionally exempt. The point is: don't assume. Check with your jurisdiction early, and check what thresholds apply to your specific project type.
The permit also distinguishes between "new development" and "redevelopment," with slightly different rules for each. If you're rebuilding more than 50% of a building's footprint, you're likely in redevelopment territory and C.3 applies to the impervious area you're replacing, what's new.
What Does Compliance Actually Look Like?
This is the part that affects your site plan the most. C.3 compliance means installing Low Impact Development (LID) measures — physical features designed to infiltrate, filter, or slow down stormwater runoff. The most common ones we design are:
- Bioretention cells (also called rain gardens or biofiltration swales): Planted depressions with engineered soil that filter runoff before it infiltrates or drains to the storm system. These are the workhorse of C.3 compliance. They require space — typically 4% to 8% of the tributary impervious area — and they need to be positioned where they can actually capture drainage.
- Permeable pavement: Porous asphalt, pervious concrete, or permeable pavers that let rain pass through the surface into an aggregate base below. Works well in parking lots and low-traffic areas where maintenance can be controlled.
- Green roofs: Vegetated roof assemblies that absorb rainfall before it becomes runoff. Useful when ground-level space is constrained, though they carry structural and maintenance implications.
- Cisterns and harvesting systems: Collected runoff stored for reuse in irrigation or toilet flushing. Less common as a primary C.3 strategy, but increasingly popular as an add-on.
The specific measures required depend on your project's size, soil conditions, depth to groundwater, and the treatment and flow-control calculations your engineer runs. We design all of these at Calichi, and the right answer is almost always a combination — not one single feature.
what's a Stormwater Control Plan?
The Stormwater Control Plan, or SCP, is the document that proves your project meets C.3 requirements. Think of it as the engineering report that sits behind all of those bioretention cells and permeable pavement sections on your civil drawings.
A complete SCP typically includes:
- A site map showing all impervious areas and drainage patterns
- Calculations demonstrating that your LID measures provide the required treatment volume and flow control
- A description of each stormwater control measure, including sizing, materials, and how it drains
- An Operations and Maintenance (O&M) Plan — which we'll get to in a moment
- A completed C.3 Data Form (a checklist required by most Bay Area municipalities)
The SCP is submitted with your grading permit application and reviewed by the city's Public Works or Engineering department. You can't get a grading permit without an approved SCP. Full stop.
This is why C.3 needs to be on your radar at the schematic design phase, not after your site plan is locked. The bioretention areas need to be where the drainage goes. If you don't leave room for them on the site plan early, you're either redesigning the parking layout or you're spending money on a more complicated (and expensive) underground system.
We've helped dozens of project teams thread this needle during site planning. It's a much easier conversation at 30% design than at permit submittal.
The O&M Agreement: The Part Nobody Reads Until It's Too Late
Here's the piece that surprises developers most: the Operations and Maintenance Agreement.
Once your project is built, someone has to maintain those bioretention cells — pulling weeds, replacing dead plants, clearing inlet grates, and occasionally replacing the filter media. The MRP requires that this obligation be formalized in a recorded legal document that runs with the property in perpetuity.
That's not a typo. In perpetuity. The O&M agreement gets recorded with the county, and it binds every future owner of the property to maintain the stormwater features. When you sell the building, the buyer takes on the obligation. Their buyer takes it on after that.
Most municipalities also require annual self-inspection reports be submitted to their stormwater program. Some do periodic field verification. It's a real ongoing compliance obligation, not a one-time permit box to check.
If you're buying a property with existing C.3 features, ask for the O&M plan and inspection records as part of due diligence. We flag this routinely in our stormwater due diligence work.
Why Developers Should Care (Beyond Just the Permit)
The honest answer is: C.3 costs money and takes space. Bioretention cells eat into your parking count. Permeable pavement costs more upfront than standard asphalt. The SCP and O&M plan add engineering fees and a recorded encumbrance on your title.
But here's the thing — handled well, C.3 features also create real amenities. A well-designed bioretention area with good planting is landscaping that does stormwater work. Green infrastructure softens sites, provides shade, and often improves project scores under local sustainability ordinances. We've seen C.3 features transform what would have been a bleak parking perimeter into something that actually helps a project lease.
The difference between C.3 as a cost and C.3 as an asset is almost entirely a planning and design question. Get the stormwater engineer in the room early. Know your thresholds before the site plan is set. And don't let compliance drive the design — let the design integrate compliance.
That's what we do. Give us a call if you want to talk through what C.3 means for a specific project.
