The 50% impervious rule, explained by people who run the numbers for a living.
Low Impact Development (LID) is stormwater regulation that treats rain as a resource instead of a nuisance. Instead of piping runoff straight to the street, LID wants it intercepted close to where it falls, then infiltrated, filtered, or slowed down through rain gardens, permeable surfaces, and similar best management practices (BMPs).
It is a good idea for new development. It is also extra design and construction cost, which is why the first question on a rebuild is whether LID applies at all.
For LA County fire rebuilds, the trigger is simple: LID requirements apply when your post-development impervious surface increases more than 50% compared to pre-fire conditions.
What counts as impervious: roof area, driveways, concrete patios, walkways, sport courts, anything rain cannot soak through.
Here is why most Altadena rebuilds pass: your house burned down, but the roof and driveway it replaced already existed. Rebuilding a similar footprint against a pre-fire baseline usually lands under the threshold. Doubling your hardscape is what trips it: a bigger driveway, a new patio, an ADU roof where there was gravel.
The impervious number is not the only check. Maintaining the pre-fire sheet flow pattern, meaning water leaves your lot along the same path it did before, is the second condition for staying out of LID. A lot that drains the same way with similar surfaces is treated as a like-for-like replacement.
This is also the same analysis that keeps cross-lot drainage disputes and sump pump requirements off your project. One flow-rate comparison, three problems avoided. That is the hydrology analysis we run on every rebuild.
When your project exceeds the threshold, you select and size BMPs: rain gardens, permeable pavement, infiltration systems, bioswales. The BMP design integrates into the drainage plan and gets reviewed with the permit. It is very doable. It just adds cost and coordination that staying under the threshold avoids.
If you are early in design, we can tell you where your impervious number lands before your architect locks the site plan. Sometimes shifting a patio surface type is the difference between LID and no LID.
Send us the address and proposed plans. We will tell you if LID applies before it costs you anything.
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