Draining a Sideways Lot Addition When You Cannot Slope to the Rear

We talked with an architect and contractor about an Altadena fire rebuild addition. The lot slopes east to west, which puts the higher elevation at one side property line and the lower elevation at the opposite side. The existing garage sits at the higher side of the property. The addition would push back toward the lower side of the lot. Drainage plans were part of solving this one.

The Problem

The addition rear sat lower than the existing garage elevation. This created two requirements.

First, the 8 inch minimum from landscape to finished floor. Standard practice to keep water away from the foundation. The addition floor had to be at least 8 inches above the surrounding grade.

Second, the garage floor transition. The garage sits below the new addition floor, and they planned steps down with a 3ft wide path between them.

But this raised a drainage question. If you have steps down and the rear of the addition sits lower, where does the water go?

Initial Options

The team discussed several approaches.

Infiltration pit. A gravel-filled pit that allows water to percolate into the ground. LA County allows infiltration pits but requires 15ft from structures and 5ft from property lines. On tight hillside lots, those setbacks can be hard to fit.

Underground tanks. Catch water in tanks and pump it out. Works but adds cost, maintenance, and mechanical complexity.

Plastic-lined rain garden. A vegetated depression that collects and filters stormwater. Sometimes we skip the rain garden entirely and pipe stormwater directly to daylight it out.

The Better Alternative

Instead of bringing area drains and pipes down to the rear of the lot, we looked at the sideyard.

The solution: pipe down to the sideyard, run laterally to the front of the property, and daylight the pipe into the front yard. The pipe exits at a lower elevation and water flows out by gravity, no pump required.

This approach works when the sideyard has enough slope to carry water toward the front, the front yard has capacity to absorb or redirect the discharge, and the drainage does not change the pre-fire flow patterns going onto adjacent properties.

Why This Wins

No mechanical system means no pump to maintain, no electricity to pay, and no failure point if the pump breaks. Gravity does the work.

For a sideways lot where rear drainage is difficult, looking at the sideyard and piping to the front is often the simplest solution. It requires less pipe, fewer fixtures, and keeps everything above grade where it can be inspected and maintained. The pre-fire and post-fire drainage patterns stay the same or improve.

Have a sideways lot in Altadena with drainage questions? Get in touch before your architect finalizes the plans. We can help identify the drainage solution early in the design process.