LA County Drainage Plan Requirements

The Short Answer

LA County Code section 22.130.160 is the basis for drainage plan requirements on residential lots. Whether your project needs one depends on topography and drainage patterns, not on a blanket rule. Lots with significant slope, drainage that crosses property lines, or downstream neighbors close by will need a drainage plan.

For Altadena fire rebuilds, most lots we see need a drainage plan. Fewer need LID compliance, and fewer still need a sump pump, which is the expensive item worth understanding early.

The 50% Impervious Surface Rule (LID)

Low Impact Development (LID) requirements trigger when your post-development impervious surface increases more than 50% compared to pre-fire conditions. Impervious surface means anything water cannot soak through: roof area, driveways, concrete patios, walkways.

Two things keep you under LID:

• Keep new impervious area within 50% of what existed pre-fire
• Maintain the pre-fire sheet flow pattern: water leaves the lot the same way it did before

A house that burned down already had a roof and driveway. Rebuilding similar footprints usually stays under the threshold. Doubling your hardscape is what trips it. More on LID compliance here.

Cross-Lot Drainage: Your Two Options

If your project changes drainage at a shared property line, LA County gives you two paths (per their recovery FAQ):

Option 1: Offsite drainage acceptance covenant. Any impact to the adjacent property must be mitigated onsite, or you get a covenant signed by the adjacent property owner and recorded against their title. This means getting a neighbor to sign a legal document. It works, but it is slow and depends on the neighbor.

Option 2: Negligible net effect finding. If the pre- and post-construction drainage net effect at the property line is negligible (as determined by the architect or engineer of record), the findings are reviewed and, if accepted, the statement is placed on the plans for permanent record. This is the path we pursue on most projects, because we can usually show the rebuild does not increase flow to the neighbor.

What Plan Check Looks For

• Pre-fire vs post-development flow rates at the property line
• Impervious surface comparison (the 50% rule)
• How roof, driveway, and patio runoff is routed
• Whether a sump pump is justified (this is where a hydrology analysis matters)

LA County does not require a hydrology report as a standalone document, but plan checkers use hydrology to justify sump pump requirements. Showing post-development Q does not exceed pre-fire Q is how a pump requirement gets removed. See how that works.

Frequently Asked Questions