If you’re developing a project anywhere in the San Francisco Bay Area, C.3 stormwater requirements probably apply — and probably control more of your site design than you think. C.3 is the part of the regional stormwater permit that governs what your project does with rainfall: how much treatment is required, what facilities count, where they go on the site, and how the agency confirms the design works.

This guide explains what C.3 is, what triggers it under MRP 3.0, the three threshold tiers, what gets required at each tier, and which watershed program reviews your project.

What C.3 Is

“C.3” refers to Provision C.3 of the Bay Area Municipal Regional Stormwater Permit (MRP), the regulatory document issued by the San Francisco Bay Regional Water Quality Control Board. The MRP applies to roughly 80 cities and counties around San Francisco Bay; each Bay Area jurisdiction operates under one of these permit conditions, regardless of whether you’re in Oakland, Sunnyvale, Walnut Creek, San Mateo, or anywhere else in the region.

Provision C.3 specifically addresses new development and redevelopment stormwater performance requirements. The provision sets thresholds: above certain impervious-surface sizes, your project must include facilities that retain, treat, and slow down stormwater before it leaves the site.

The current permit version is MRP 3.0, adopted by the Regional Board in May 2022 and effective July 2022. MRP 3.0 dropped the regulated-project threshold significantly, expanded the watershed programs covered, and introduced new requirements for trash, mercury, and PCBs control.

The Three C.3 Threshold Tiers Under MRP 3.0

Your project falls into one of three tiers based on how much new or replaced impervious surface you create:

Tier	Trigger	What’s required
Tier 1 Small Projects	2,500 to 4,999 SF of new or replaced impervious surface (single-family, duplex, etc.)	Site design measures only: pervious pavement, disconnected downspouts, green roofs, rainwater harvesting. No engineered treatment required.
Tier 2 Regulated Projects	5,000 to 9,999 SF of new or replaced impervious surface (most small commercial and multifamily)	Stormwater Control Plan + treatment of the “regulated area” using LID measures (bioretention typically). Hydromodification not required.
Tier 3 Large Regulated Projects	10,000+ SF of new or replaced impervious surface (most multifamily, mixed-use, commercial, public works)	Full C.3 package: SCP, treatment sizing, hydromodification (HMP) where applicable, Operation & Maintenance agreement, agency-stamped construction drawings.

The threshold dropped from 10,000 SF (under MRP 2.0) to 5,000 SF (under MRP 3.0) for the Regulated Project tier. That change pulled thousands of formerly-exempt small commercial and multifamily projects into the regulated bucket. If you’ve worked on Bay Area development before 2022 and assume your 7,000-SF project is exempt, it isn’t anymore.

What a Stormwater Control Plan (SCP) Contains

For Tier 2 and Tier 3 projects, the Stormwater Control Plan is the central deliverable. It’s a multi-section narrative + exhibits document that demonstrates compliance:

• Project description — site, scope, drainage management areas (DMAs)
• Source control measures — trash capture, pollution prevention design, restaurant grease control, vehicle wash limits
• Site design measures — minimizing impervious area, preserving natural drainage, disconnecting impervious areas
• Treatment sizing calculations — using either the volume-based or flow-based method, demonstrating treatment of the 80% capture volume
• LID treatment facility design — bioretention, flow-through planters, pervious pavement, harvest-and-use systems
• Hydromodification analysis (Tier 3 only, where applicable) — modeling site discharge against pre-project baseline using BAHM or watershed-specific tools
• Operation & Maintenance plan — ongoing inspection schedule, responsible party, recorded O&M agreement

The SCP gets reviewed by the local agency’s public works or stormwater compliance staff, with technical assistance from the watershed program (more on that below).

The Watershed Programs — Who Reviews Your Project

Bay Area MRP compliance is delegated to countywide stormwater programs that share permit administration with their member agencies. Each program has its own technical guidance, sizing tools, and submittal requirements:

Program	Member counties	Key tools
ACCWP (Alameda Countywide Clean Water Program)	Alameda County and member cities (Oakland, Berkeley, Hayward, Fremont, etc.)	BASMAA C.3 Technical Guidance, ACCWP-specific O&M templates
CCCWP (Contra Costa Clean Water Program)	Contra Costa County and member cities	Stormwater C.3 Guidebook, BAHM hydromodification tool
SCVURPPP (Santa Clara Valley URPPP)	Santa Clara County and member cities	Hydromodification Plan, C.3 Handbook, treatment sizing calculator
SMCWPPP (San Mateo Countywide Water Pollution Prevention Program)	San Mateo County and member cities	SMCWPPP C.3 Guidebook
FSURMP (Fairfield-Suisun Urban Runoff Management Program)	Fairfield, Suisun City, Vacaville	FSURMP-specific submittal requirements
VSFCD (Vallejo Sanitation & Flood Control District)	Vallejo, American Canyon	VSFCD-specific submittal requirements

The first thing a civil engineer does on a Bay Area project is identify which watershed program applies, then design to that program’s guidance — not generic state-level minimums. ACCWP’s bioretention sizing approach is not identical to SCVURPPP’s; sub-base specs and plant lists differ. Designing to one program’s guidance and submitting to a different program’s reviewer is the most common cause of plan check rejections.

Treatment Sizing — The 80% Capture Volume

The core C.3 sizing requirement is treatment of the 80% capture volume — sized to capture and treat the runoff from the smallest 80% of storms, by volume, in an average rainfall year. There are two equivalent sizing methods:

• Volume-based — size the facility to capture a specific runoff volume from the contributing area, using watershed-program-specific runoff coefficients and design storm depths.
• Flow-based — size the facility to treat the runoff from a specific design storm intensity using standard hydraulic capacity calculations.

For most Bay Area bioretention designs, the volume method results in a facility footprint roughly 4 percent of the contributing impervious area. So a 20,000-SF parking lot needs a treatment facility of about 800 SF. That facility footprint is what drives the site planning decision — and what most projects underestimate at the concept stage.

Hydromodification (HMP) — Tier 3 Add-On

For Tier 3 projects in certain watersheds, the project must also meet hydromodification requirements: site discharge rates and durations must mimic pre-project conditions to prevent downstream channel erosion. HMP applies when:

• Project is 10,000+ SF impervious (or 1+ acre disturbed in some programs)
• Project drains to a watercourse with documented hydromodification susceptibility
• Project is not exempt under HMP applicability criteria (in-fill, urbanized, low-effect)

HMP analysis is done with continuous-simulation modeling tools — BAHM (Bay Area Hydromodification Model) for ACCWP/CCCWP, or watershed-specific tools for other programs. The modeling typically requires either expanding the bioretention size beyond C.3 minimum or adding a separate detention component.

What Triggers Common Plan Check Rejections

The same C.3 issues come up repeatedly in plan check. Avoiding them up front saves a review cycle:

• Wrong watershed program guidance — designing to MRP minimums instead of the watershed-specific handbook.
• Ignoring “replaced impervious” — assuming a redevelopment is exempt because it doesn’t add net impervious.
• Mis-sized bioretention — using a runoff coefficient that doesn’t match the watershed’s technical guidance, or applying flow-based sizing where the program prefers volume-based.
• Missing self-treating credits — not claiming Provision E.12 self-treating areas (landscape, undisturbed natural areas) that reduce the regulated area before treatment.
• No O&M agreement — submitting a stormwater control plan without the recorded O&M agreement, which most programs require before construction starts.
• Sub-base specs that don’t match the manual — using generic engineered soil mix instead of the watershed program’s prescribed bioretention soil specification.
• Plant list mismatches — using plant species not on the program’s approved list for bioretention.

What This Means for Your Project Schedule

C.3 design isn’t something to bolt on at 90% improvement plans. The treatment facility footprint drives parking layout, drainage flow paths, and grading. Get C.3 into the project at the conceptual site planning stage. Realistic timeline:

• Concept stage: identify watershed program, calculate rough treatment area (~4% of impervious), reserve location on site plan.
• Tentative map / entitlement: preliminary stormwater control plan with sizing calculations and concept facility design.
• Improvement plans (50%): full SCP with bioretention details, plant list, soil spec, underdrain design.
• Plan check: 1–3 review cycles, typically 30–60 days each.
• Construction: SWPPP / construction stormwater compliance, certification of facility installation.
• Pre-occupancy: recorded O&M agreement, agency sign-off.

For most Bay Area projects, expect 60–90 days from SCP submittal to first plan check response and 120–180 days total through to agency approval, including resubmittals.