Three pages. That's roughly what Provision C.3 takes up in the Municipal Regional Stormwater Permit., those three pages have more influence over a Bay Area site plan than almost anything else in your permit stack — more than setbacks, more than FAR, sometimes more than the fire code.
If you're building or redeveloping anything significant in the Bay Area and haven't built C.3 compliance into your site plan from the start, you've already made things harder than they need to be. This post walks through what C.3 actually requires, why it's structured the way it's, and how we integrate it into projects so it stops being a cost driver and starts being just part of the design.
what's Provision C.3, Exactly?
The Municipal Regional Stormwater Permit — the MRP — is issued by the San Francisco Bay Regional Water Quality Control Board (SF Bay RWQCB). It covers 76 cities and counties across nine Bay Area counties. The current version, MRP 3.0, was issued in 2022. Before that, MRP 2.0 ran from 2015 to 2022. The original MRP 1.0 dropped in 2009.
Each iteration has been stricter than the last. That trend isn't slowing down.
Provision C.3 is the section of the MRP that reaches down to individual development and redevelopment projects. The rest of the permit governs how municipalities run their stormwater programs. C.3 is the piece that shows up on your grading permit review comments.
In plain terms: if your project creates or replaces 5,000 square feet or more of impervious surface (10,000 SF for single-family homes), you're responsible for managing the stormwater quality and quantity impacts from that impervious area on your own site. The city's storm drain isn't your stormwater treatment system anymore. Your site is.
The Three Requirements That Actually Matter
C.3 has a lot of provisions, there are three that shape your site plan most directly: source control (C.3.i), site design (C.3.c), and treatment (C.3.d). They're listed in the order the permit presents them, which is also roughly the order they should come up in your design process.
Source Control (C.3.i): Stop Pollutants Before They Hit the Pavement
Source control measures are design features and operational practices that prevent pollutants from reaching stormwater in the first place. They're the least glamorous part of C.3, they're required on every regulated project.
On the design side, this means things like: covering trash enclosures and loading docks to keep rain off of them, routing rooftop equipment drains to sanitary (not storm), providing wash-down drains for vehicle areas that connect to sanitary, and routing parking lot drains away from high-pollutant zones before they reach your treatment BMPs.
These aren't optional. The Stormwater Control Plan (SCP) you submit with your grading permit application has to document source control measures for every applicable pollutant source on the site. BASMAA — the Bay Area Stormwater Management Agencies Association — publishes the IMP (Integrated Management Practices) Fact Sheets that define what's required for each source type. Your civil engineer and your architect both need to be talking about this.
Site Design (C.3.c): LID First, Everything Else Second
This is where C.3 starts to have real opinions about your grading plan.
C.3.c requires that projects apply Low Impact Development (LID) site design measures to minimize impervious surface and direct runoff to pervious areas wherever feasible. The MRP establishes a clear hierarchy: infiltrate first, harvest and reuse second, biotreat third. You work down the list. If infiltration isn't feasible at your site (poor soils, high groundwater, contaminated fill), you document why, and you move to the next option.
What this means in practice: your grading plan needs to direct drainage toward planted areas and away from concentrated discharge points wherever the grades allow it. Parking lots should sheet drain to vegetated buffers. Building downspouts should discharge to landscaped areas, not straight to the storm drain. Impervious area should be minimized where it doesn't serve a program need.
On a typical Bay Area commercial or multifamily project, C.3.c doesn't usually require drastic changes to the program., it does require that your civil engineer and landscape architect are coordinating grading and planting from the start — not handing off after the grading plan is finished.
Treatment (C.3.d): Size It, Place It, Prove It
This is the one that takes up the most space on your drawings.
C.3.d requires that regulated projects install stormwater treatment controls — physical BMPs — sized to treat the runoff from the regulated impervious area. For most Bay Area projects, the sizing standard is the 80th percentile storm event, which translates to a water quality design storm of roughly 0.2 to 0.6 inches per hour depending on location and watershed.
The LID BMP hierarchy applies here too. You're trying to infiltrate first. If that's off the table, you're harvesting or biofiltrating. The most common BMPs we design are:
- Bioretention cells — engineered planted depressions with a specified soil mix and underdrain. Workhorses. Sized at roughly 4% to 8% of tributary impervious area, depending on your jurisdiction's IMP sizing method. They need to go where the drainage goes, which is a grading question, not a landscaping question.
- Flow-through planters — bioretention in a raised planter box, typically used at building edges or where infiltration isn't feasible and ground-level space is tight. Common on urban infill projects.
- Pervious pavement — porous asphalt, pervious concrete, or permeable pavers over a reservoir base. Works well in low-traffic parking areas. Maintenance-sensitive. Make sure your property manager understands what they're agreeing to before you design it in.
- Tree wells — silva cells or similar structural soil systems under paving that allow tree root growth while filtering runoff. Useful in streetscape and urban parking contexts. Also counts toward C.3 treatment credit in some jurisdictions.
The SCP has to include calculations showing that each BMP is properly sized for its tributary area and that the total treatment coverage meets the C.3 requirement. We run these calculations using the BASMAA IMP sizing worksheets, adjusted for the specific countywide program your project falls under.
Wait — Which Countywide Program Applies to My Project?
Here's something that trips up project teams who work across county lines: the MRP is a single permit, each county has its own Countywide Program that implements it. Same requirements, different forms, different review contacts, sometimes different sizing methods or local thresholds.
The four you'll encounter most often in the Bay Area:
- CCCWP — Contra Costa Clean Water Program. Covers Contra Costa County cities.
- ACCWP — Alameda Countywide Clean Water Program. Covers Alameda County cities.
- SCVURPPP — Santa Clara Valley Urban Runoff Pollution Prevention Program. Covers Santa Clara County cities.
- SMCWPPP — San Mateo Countywide Water Pollution Prevention Program. Covers San Mateo County cities.
The difference matters at permit submittal. The SCP data forms, the IMP sizing worksheets, and sometimes the accepted BMP list are specific to each program. Using the wrong jurisdiction's forms is a quick way to get a completeness rejection. We track this so you don't have to.
Hydromodification Management: The Requirement That Sneaks Up on Projects
Treatment isn't the only C.3 requirement with a sizing calculation. Hydromodification management (HM) applies to projects that discharge to erosion-sensitive streams and waterways — which covers a lot of Bay Area sites.
HM requires that your project not increase peak stormwater runoff rates or flow durations above pre-project conditions for a range of storm events (typically from the 10th to 50th percentile flow, up to the 10-year storm, depending on the applicable flow-duration standard). If your site is adding impervious area and discharging to a regulated waterway, you likely need either an HM facility (a detention basin or underground cistern with a controlled outlet) or you need to demonstrate an exemption.
HM adds cost and site area. We evaluate HM applicability early in the site planning phase because it's the kind of requirement that can force a meaningful redesign if it surfaces at permit check. A site that looked perfectly sized at 30% design can get tight fast when you add a 2,000-gallon underground detention vault.
The O&M Agreement: The Fine Print That Runs With the Property
After your project is built and your grading permit is closed out, C.3 isn't done with you. Every regulated project is required to record an Operations and Maintenance (O&M) Agreement with the county — a legal document that obligates the property owner to maintain the stormwater treatment features in perpetuity.
That means annual inspections, regular vegetation maintenance in bioretention cells, periodic media replacement, inlet cleaning, and submission of annual self-inspection reports to your municipality. Some countywide programs do periodic field audits. It's an ongoing compliance obligation tied to the deed.
If you're acquiring a property with existing C.3 features, review the recorded O&M agreement and ask for inspection records before closing. We catch deferred maintenance issues in stormwater due diligence reviews that would otherwise become the new owner's problem.
What Happens If You Ignore C.3 Until 90% CDs?
We get calls about this more than we'd like.
The scenario is consistent: the project team locked in the site plan early, nobody flagged C.3 requirements, and now there are 90% construction documents with no room for bioretention, no routing strategy for drainage, and a grading permit submittal on the calendar. Fixing it means one or more of the following: redesigning the parking layout to free up space for BMPs, adding expensive underground LID infrastructure (flow-through vaults, underground bioretention chambers), delaying the permit submittal, or — in the worst case — reducing the program to get under a threshold.
All of those cost more than getting us into the project at site planning. That's not a pitch — it's just math. An LID-informed grading plan at 30% design costs the same to draw as one that ignores stormwater. The difference is that one of them goes through permit without revision comments on the SCP.
How We Handle C.3 on Our Projects
When we're brought in at the start, C.3 compliance is woven into the grading and drainage design from the first site plan iteration. We identify the regulated impervious area, determine which countywide program applies, evaluate infiltration feasibility based on available soils data, and rough out the BMP locations and sizing before the parking layout is frozen. By the time we're at 60% CDs, the SCP is in progress and the BMP sizing is confirmed.
When we're brought in mid-project — which happens — we assess where the design is and identify the lowest-cost path to compliance. Sometimes that's adding planters at the building perimeter. Sometimes it's replacing a section of parking lot with pervious pavement. Sometimes it requires a harder conversation about the site plan.
Either way, we run through the SCP and O&M documentation, coordinate with the applicable countywide program, and manage the stormwater review comments through construction administration so the project team isn't chasing stormwater RFIs in the field.
If you've got a project coming up in the Bay Area and you're not sure where C.3 fits into your permit timeline, give us a call. We'll tell you what applies, what it costs to get there, and how to keep it from running the site plan.
