Does My Project Trigger C.3? The Square Footage Thresholds That Matter
The short answer: if your project creates or replaces 5,000 square feet or more of impervious surface in the Bay Area (10,000 SF for single-family homes), C.3 stormwater requirements apply under MRP 3.0. That threshold comes from Provision C.3 of the Municipal Regional Stormwater Permit (MRP) — the permit that governs what triggers C.3 stormwater requirements across 76 Bay Area cities and counties. Gas stations, auto service shops, and restaurants hit a lower bar at 5,000 square feet. Single-family homes are usually exempt unless you're subdividing into 10 or more lots. Road projects follow the same 10,000 square foot rule for new or replaced pavement.
That's the cheat sheet. The rest of this post is the fine print that determines whether you're actually in or out — and what to do if you're right on the edge.
What Counts as Impervious Surface?
Before we get into thresholds, let's make sure we're measuring the right thing. Impervious surface is anything that prevents rain from soaking into the ground: concrete, asphalt, rooftops, compacted gravel, and most pavers. Landscaping and permeable pavement (installed correctly and counted properly) aren't impervious.
When you're calculating your project's impervious area, you add up all of that — the building footprint, the parking lot, the drive aisles, the trash enclosure pad, the sidewalks. Everything that sheds water instead of absorbing it.
The MRP cares about two flavors of impervious area: new (didn't exist before) and replaced (existing impervious you're demolishing and rebuilding). Both count toward your threshold. That second category is where redevelopment projects routinely get surprised.
The Redevelopment Trap: The 50% Rule
Here's the one that catches developers off guard more than any other.
If you're tearing down an existing building and rebuilding, you're adding new impervious area — you're replacing old impervious area too. Under the MRP, if the impervious surface you're replacing equals 50% or more of the site's existing total impervious footprint, the entire site gets treated as a new development project for C.3 purposes.
Walk through an example. You've got an old strip center on a 1-acre site. The existing footprint is 12,000 square feet of building and 18,000 square feet of parking — 30,000 square feet total impervious. You're planning to demolish the building and rebuild it at 14,000 square feet, touching only the building pad, not the parking lot.
You're replacing 12,000 square feet of impervious (the old building). That's 40% of the 30,000 square feet total. You're under the 50% threshold — so C.3 treatment requirements apply only to the new and replaced areas, not the whole site., change your plans to also resurface the parking lot, and suddenly you're replacing 30,000 of 30,000 square feet. The whole site triggers full C.3 treatment requirements.
The math matters. We run this calculation in the first hour of every site planning conversation on a redevelopment project, because it affects the budget before a single line gets drawn.
Special Project Types With Lower Thresholds
Three categories of land use have a lower trigger under the MRP because of their higher pollutant loading — meaning rain that falls on these sites picks up more contamination than a typical parking lot:
- Automobile service facilities (oil change shops, car washes, body shops, service bays): 5,000 square feet of impervious surface triggers C.3.
- Gas stations: 5,000 square feet, or any project that creates a new fuel dispensing area.
- Restaurants: 5,000 square feet. Grease, food waste, and outdoor dining areas are the reason.
If you're doing a quick-service restaurant with a drive-through on a 12,000 square foot pad, you're triggering C.3 at the 5,000 square foot restaurant threshold — you're also likely subject to enhanced source controls under Provision C.3.i. Source control requirements are separate from treatment requirements and kick in at lower thresholds. More on that in a moment.
Single-Family Homes: Usually Exempt, Not Always
Individual single-family home projects are generally exempt from C.3 treatment requirements. You can build a 4,000 square foot house on a single lot without triggering a Stormwater Control Plan.
The exception: subdivisions of 10 or more lots. If you're subdividing land for residential development, the subdivision itself is evaluated as a project. The aggregate impervious area across all lots, streets, and common areas gets added together, and if it hits 10,000 square feet (which it will for any subdivision of meaningful size), C.3 applies to the project as a whole.
That means the civil engineer designing the subdivision has to account for stormwater treatment at the tract level — often through shared bioretention features, detention basins, or low-impact-development conditions of approval built into the tentative map.
Road Projects
Public road improvement projects follow the same 10,000 square foot threshold for new or replaced impervious surface. Resurfacing an existing roadway without widening or rebuilding the base typically doesn't count as replacement under MRP definitions —, a full reconstruction does.
Municipal and county agencies doing capital improvement work in the Bay Area are subject to the MRP just like private developers. Their stormwater programs (CCCWP in Contra Costa, ACCWP in Alameda, SCVURPPP in Santa Clara, SMCWPPP in San Mateo) all operate under the same MRP, each has its own submittal forms, design checklists, and review timelines. If you're managing a public works contract in the Bay Area, make sure you know which countywide clean water program you're filing under — the forms aren't interchangeable.
C.3.i, C.3.c, and C.3.d: Three Provisions, Three Thresholds
When people say "C.3 compliance," they usually mean the treatment requirement — the bioretention cells and permeable pavement., C.3 actually has three distinct tiers, and they apply at different scales:
- C.3.i — Source controls: Basic housekeeping measures that prevent pollutants from reaching storm drains. Covered trash enclosures, drain inlet screens, outdoor storage area grading. These apply to a broad range of projects, including some that don't hit the treatment threshold. If you're doing anything with food service, fueling, or outdoor material storage, source controls are almost certainly required regardless of project size.
- C.3.c — Site design: Design measures that reduce the generation of runoff in the first place. Directing roof drains to landscaping, using permeable paving in low-traffic areas, minimizing disturbed area. Required for most regulated projects and integrated into the Stormwater Control Plan.
- C.3.d — Treatment: The full stormwater control plan requirement. Bioretention cells, flow-through planters, infiltration trenches, or other treatment measures sized to handle the design storm from your regulated impervious area. This is what most people mean when they say C.3.
Understanding which provisions apply to your project isn't academic — it changes your site plan, your engineering scope, and your construction budget. We walk through this in every stormwater management engagement we take on, usually before the first site plan revision.
Just Under the Threshold? Cities Can Still Require BMPs
Here's the part nobody mentions in the permit handouts: landing at 9,500 square feet of impervious area doesn't automatically mean you walk away with no stormwater obligations.
Most Bay Area cities reserve the right to impose Best Management Practices (BMPs) on projects that are below the MRP thresholds, either through their local municipal code, conditions of approval, or discretionary review. If your project is going through a planning commission or design review board, stormwater performance conditions can appear on the approval even if you're technically exempt from the MRP treatment threshold.
The practical advice: if you're within 15–20% of any threshold, go to a pre-application meeting with the city's Public Works or Engineering department before you finalize your site plan. Ask directly what stormwater conditions they anticipate. You're better off knowing early than discovering a bioretention cell requirement after your parking count is already tight.
We go to those pre-app meetings regularly, and they almost always surface something worth knowing. Give us a call if you want a second set of eyes on where your project stands before you head in.
