The dirt work under every backyard unit
Every ADU in San Diego starts as an excavation project. Before a single wall frames up, somebody has to cut a level pad, compact it to spec, trench sewer, water and power back to the connections and truck away whatever soil the cut displaced. That package runs $8,000 to $25,000 on our published cost guide, and the spread is your lot talking: slope, access and how far the utilities have to travel. The city permitted more than 1,100 ADUs in a single recent year, so this scope is now the center of what we do.
Why the dirt decides the budget
ADU budgets blow up underground, not above it. The unit itself prices predictably per square foot; the site does not, because no two backyards share a slope, a soil or a path to the sewer. A flat El Cajon lot with alley access preps near $8,000. A benched pad on a Mount Helix hillside with a ninety-foot sewer run can honestly reach $25,000, and an owner who learns that after signing a construction contract learns it painfully. We quote the dirt first, in writing, so the rest of the project stands on a known number.
The sewer connection is usually the wildcard. Gravity wants the new unit’s lateral to fall continuously to the connection point, and where the yard refuses, the choices are deeper trenching or an ejector pump, each with real costs we lay out plainly. Water and power runs are simpler, but they share trenches where code allows and save money doing it, which is the kind of detail a dirt-first plan captures.
How the site package runs
- 1
Walk it before you design it
The free site walk tells you which corner digs cheapest and what the utilities will cost. Bring your designer early or bring us your plans; both work.
- 2
One written package number
Pad, trenches, export and compaction, itemized against the public table.
- 3
811 marks and the cut
DigAlert marks first, then the pad cut and compaction in lifts to the geotechnical spec.
- 4
Trenches while the machine is there
Sewer, water and power runs opened, bedded and closed in the same mobilization.
- 5
Paper and handoff
Compaction reports for the permit file and a pad your builder can start on the same week.
The city’s ADU rules, fee structures and permit pathways are published at sandiego.gov, and they are worth reading before any design contract.
ADU dirt questions, answered
What does ADU site prep actually include?
The compacted building pad cut to your plans, the utility trenches for sewer, water and power back to the connection points, export of the displaced soil and the compaction testing your permit requires. Our published package range is $8,000 to $25,000, with flat open lots at the bottom and hillside cuts with long utility runs at the top.
When should the excavation contractor get involved in an ADU project?
Before your design is final. A site walk early tells you which corner of the lot digs cheapest, whether the slope demands engineering and what the utility runs will honestly cost. Owners who price the dirt first avoid the classic ADU budget surprise, which almost always lives underground.
Can you trench utilities under an existing driveway or patio?
Yes, by cutting and restoring the hardscape or, where the run and soil suit it, boring beneath it. Surface restoration is a line on the quote rather than a surprise, and sometimes rerouting the trench four feet to miss the patio is the smartest money on the project.
Does the pad come with the compaction paperwork the city wants?
Yes. Pads are built in lifts to the geotechnical spec and tested by your soils engineer at the right stages, and the compaction reports go into your permit file. A pad without the paper is not a pad the city will build on.
An old pool in the ADU footprint is common enough that the pool removal page covers the combined project, and Cabrillo Excavation prices the pair in one visit.
Price the dirt before the design locks
Send the address and what you are planning. The site walk is free and the number comes back in writing.