The ADU Timing Question Every Altadena Fire Rebuild Owner Should Ask Before Plans Are Approved

Every few weeks we get a call from someone who is already mid-construction in Altadena. The house is going up, the permit is in plan check, and then someone mentions they want to add an ADU in the back.

This is the moment where the math changes.

If you are reading this and you are in Altadena doing a fire rebuild, this post might save you $20,000 or more. Here is why.

The Short Version

The direct answer: If you plan to add an ADU to your Altadena fire rebuild property, include it in your initial drainage plans or grading plans. LA County is currently being flexible on pump requirements for fire rebuilds where post-fire impervious surface and Q flow match pre-fire levels. But this leniency has a time limit. Waiting until after your certificate of occupancy could trigger a $15,000 to $20,000 sump pump requirement, plus shipping, excavation, pipe runs to the street, right of way work, and potentially a new curb.

LA County often requires pumps on Altadena fire rebuild projects. But there are ways to get around that requirement. On projects where we can show the pre-fire and post-fire impervious areas are roughly the same, and that the Q flow is similar, they have not been requiring pumps. That leniency has a clock on it.

If you are planning to add an ADU later, doing the extra engineering calculations now while the county is being flexible could mean the difference between avoiding a pump requirement and getting stuck with one you have to maintain forever.

Why Adding an ADU to Your Altadena Drainage Plans Matters

An ADU adds impervious surface. More roof, more concrete, more driveway. When you add impervious surface, you change how much water the site has to handle.

LA County looks at this through a lens of pre-fire versus post-fire conditions. They want to make sure your rebuild is not sending more water to your neighbors than existed before the fire.

For most fire rebuilds, we calculate the pre-fire impervious surface from satellite imagery and compare it to what is proposed. If your post-fire impervious is the same or less, the drainage pattern stays similar and plan check is usually satisfied with rain gardens.

But if you add impervious surface later, the county will look at your project under different standards. At that point, the leniency window may be closed. They might require a pump, a pump covenant, or street work.

We have not seen the county formally announce this is changing. But we handled similar projects after the 2018 Woolsey Fire in Malibu, where City of Malibu plan check had their own emergency standards. We know how these things go. The emergency standards that came after the Eaton Fire will not last forever.

What Happens If You Add an ADU After Your Fire Rebuild is Approved

If you build your house, get your certificate of occupancy, and then come back to add an ADU, LA County will likely treat that as a new project under current standards. Those standards will probably require a sump pump, a pump covenant recorded against your property, and a street plan showing improvements in the public right of way.

Here is what that actually costs beyond the pump itself. The pump equipment runs $15,000 to $20,000. Then you have to ship it, which is a few thousand dollars. You have to dig a pit, probably 6 feet deep for a 4-foot diameter tank. You need a separate hole for the shutoff valve vault. Pipe runs from the pump to the front of the property, into a 2-foot by 2-foot catch basin. Then there is right of way work to build an 18-inch parkway drain through the curb face and potentially build a new curb along your property frontage.

None of this is cheap, and none of it is optional once they require it.

We do whatever we can think of to get around needing a pump. That is why we push for including future ADU calculations in the initial plans. Adding this line item to the engineering documents early could avoid a massive expense later.

The Math: How LA County Evaluates Pre-Fire vs. Post-Fire Drainage

In the hydrology report, we compare pre-fire satellite conditions to post-fire proposed conditions. If your impervious surface increases beyond certain thresholds, the county wants to see how you are handling the additional runoff.

For Altadena lots, the topography generally does not change after a fire. The pre-fire drainage pattern is usually a good reference point. We model how water moved across the site before, and we design the drainage to maintain that same pattern after.

When an ADU is added to the initial plans, we can show the county the full picture. We include the ADU in the impervious surface calculations and design the rain gardens to handle the increased volume. As long as the post-development runoff does not exceed pre-fire volumes, a pump is usually not required.

If you wait and add an ADU later, we have to show the county that same math, but now it is under scrutiny and the rules may have changed.

Our Recommendation: Talk to Your Civil Engineer Before Plans Are Finalized

If you think there is any chance you will add an ADU to your Altadena property, talk to your civil engineer about it before your plans are finalized. It is a line item on the engineering, not a separate set of plans. But it has to be in the documents from the start.

We do this for most of our Altadena clients. Asking us to look at your site and tell you whether including the ADU makes sense is free. We only charge if we take on the engineering work. But if we do take on the work, it is a small addition that could avoid a massive expense later.

For Altadena fire rebuild projects, we can look at your specific site and tell you whether including the ADU in your initial plans makes sense for your situation. No obligation. Just useful information if you are trying to figure out what you actually need.