My Site Doesn’t Drain: What’s Causing It and Who Fixes It

If your site isn’t draining water, you have a stormwater compliance problem. In California, every development—from a single-family remodel to a 50-unit apartment complex—must meet on-site stormwater retention or infiltration standards under the California Stormwater Quality Association (CASQA) Post-Construction Requirements and local municipal codes. We’re talking about capturing and treating the first inch of rainfall (or more, depending on your Water Quality Volume threshold) before it leaves your property. If your site can’t drain, you’re either exceeding those thresholds, or your drainage design is incomplete.

The root causes are almost always one of three things: (1) your soil doesn’t infiltrate fast enough (percolation rate too low), (2) your site is too small or too impermeable to store the required volume, or (3) you don’t have an approved alternative like an underground retention basin, bioretention, or off-site mitigation. This is a civil engineering and geotechnical problem—and it requires either a soil test, a design fix, or both. Let’s walk through what’s required, where exemptions exist, and how to move forward.

What does California’s stormwater code actually require?

Start with the California Building Code (CBC Title 24) Chapter 5, which references the California Green Building Standards (CBSC) Tier 1 and Tier 2 requirements. These require on-site stormwater retention for the 85th-percentile storm event (roughly the first inch of rainfall in most of Northern California). Locally, your city enforces the CASQA Post-Construction Requirements (MCM 6.2) and may have its own additional standards.

In the Bay Area, Oakland, Berkeley, and San Jose have stricter thresholds than the state minimum. Oakland, for example, requires on-site capture and treatment of stormwater runoff from the 95th-percentile 24-hour storm event for most new and redevelopment projects—that’s roughly 1 to 1.5 inches depending on your rainfall zone. If your site can’t meet that, you need an exemption, a variance, or off-site mitigation.

The key document you need is a Phase II Environmental Site Assessment (ESA) with soil percolation testing (ASTM D3385 or D7928). This tells you whether your soil can infiltrate at all, and at what rate. If your measured infiltration rate is less than 0.5 inches per hour, on-site infiltration isn’t feasible—you’ll need storage (retention) or treatment (bioretention, permeable paving, green infrastructure).

When is on-site drainage NOT required?

Exemptions exist. If your project disturbs less than 1 acre of soil and you’re not in a sensitive watershed, you may qualify for a small-project exemption under some municipalities. Also, if you can demonstrate that your site is underlain by bedrock, hardpan, or clay with a measured infiltration rate below 0.5 in/hr, you’re exempt from infiltration-based requirements—but you’re not exempt from retention or treatment. You still have to capture and slow that stormwater runoff.

The Subdivision Map Act (Government Code §66426) also provides some relief. If you’re creating fewer than four parcels and the project doesn’t trigger CEQA, you may have lighter-touch stormwater requirements. But check with your local planning department first—exemptions vary wildly between Oakland, San Francisco, Fremont, and other Bay Area cities.

What if your soil won’t percolate?

This is the most common scenario. Your soil test comes back at 0.2 in/hr or worse. Now you have three design paths:

• Retention basin (underground or surface): Size it to hold the Water Quality Volume (WQV) and allow it to drain slowly over 48–72 hours, or rely on evaporation and plant uptake. This works on any site and is often the cheapest option for small projects.
• Bioretention or rain garden: A planted depression or berm that captures runoff, filters it through engineered soil, and percolates it slowly. Requires maintenance access and ongoing plant care, but meets green infrastructure goals and often qualifies for density bonuses or parking reductions under SB 1383.
• Off-site mitigation or Regional Stormwater Control: Pay into a local stormwater improvement fund, dedicate land for a regional basin, or purchase credits. This is common for infill projects in dense urban areas where on-site space is zero.

Real example: a 10-unit condo on a 0.8-acre site in Oakland

Let’s say you’re building a 10-unit residential project on a 0.8-acre (35,000 sq ft) lot in Oakland. Your impervious area (roofs, driveways, hardscape) is roughly 18,000 sq ft. Oakland’s 95th-percentile storm is about 1.2 inches. Your WQV is 18,000 sq ft × 1.2 in ÷ 12 = 1,800 cubic feet (roughly 13,500 gallons). Your soil test shows 0.3 in/hr permeability—no good for infiltration.

You design an underground retention basin (8 ft deep, 225 sq ft) to store 1,800 cu ft. Stormwater drains from roof gutters and permeable pavement into this basin over 48 hours via a restricted outlet orifice (usually a 1-inch weir). Overflow goes to the local storm drain or a green street project. Cost: $30K–$50K. Lead time: 6–8 weeks for design and permitting.

What codes and permits do you need?

You’ll need a Stormwater Pollution Prevention Plan (SWPPP) under the General Permit (CGP) if you disturb more than 1 acre or are in a sensitive area. You’ll also need a Post-Construction Stormwater Control Plan (C.3 plan) showing compliance with the Water Quality Volume standard. In Oakland, that’s submitted as part of the Stormwater and Sanitary Sewer Compliance checklist. Your civil engineer prepares both; your contractor implements erosion controls during grading.

If you’re using bioretention or a rain garden, you may also need a Maintenance Agreement (Subdivision Map Act §66477) signed by the property owner, promising to inspect and maintain the system annually. Failure to maintain voids your local permit.

How we can help

We’ve designed stormwater retention and bioretention systems for K-12 schools, multifamily housing, and commercial projects across the Bay Area. As a PE (California civil and QSD/QSP certified), Reco conducts Phase II soil testing, calculates your Water Quality Volume, and designs compliant systems that fit your site and budget. Give us a call if your site doesn’t drain—we’ll order the soil test, pull the right code references, and move you toward permitting.