ADU pad grading and site prep in San Bernardino
Shandin Excavation grades the pads and digs the trenches for detached ADUs, casitas, garages and workshops across the valley. San Bernardino has leaned into backyard units harder than any city around it, the old Fontana Farms lots and north Rialto parcels are big enough for them, and a unit next to the Loma Linda medical campus rents the week it is finished.
What the pad has to do
A pad is a level, compacted platform a foot or two bigger than the foundation footprint, set at an elevation that keeps the finished floor above the yard and sends water away from both the new unit and the main house. On a flat lot south of Baseline that is a few hours of cut, a few hours of compaction and a swale to the street. On a sloped lot under Shandin Hills, in Verdemont or in the Loma Linda South Hills it becomes a cut on the uphill side, a compacted fill on the downhill side, a retaining condition and a soils lab testing each lift.
The utility trenches
The pad is half the job. The sewer is the other half: a four inch line from the unit to either the existing house lateral or straight to the street main, with enough fall to drain. On a 1950s lot the existing lateral is often clay and often the owner replaces it and ties the ADU in during the same trench, which costs less than two separate jobs. Water and electrical usually share a trench from the main house. Utility trenching runs $20 to $65 a linear foot and the sewer page covers the lateral itself.
How an ADU pad goes
- 1
Plans and the permit
Your ADU designer draws the site plan with the pad elevation and drainage. The city issues the building permit and, when the lot slopes or drainage changes, a grading permit with it.
- 2
811 and clearing
The 811 ticket goes in two working days ahead. Fences, sheds and old concrete in the footprint come out.
- 3
Cut, fill and compact
The excavator cuts the high side and places fill on the low side in lifts, each one compacted. The soils lab or the grader tests to 90 percent relative compaction.
- 4
Swale and trenches
The drainage swale is shaped to the plan. Sewer, water and electrical trenches are dug, lines set by the plumber and electrician, and backfilled after the inspection.
- 5
Compaction report
The report goes to your builder for the foundation inspection. Then the forms go up.
Rules on size and approval come from the state; the California HCD ADU page is the reference. Grading triggers and compaction standards come from each city, listed on the permits page.
Price the pad and the trenches
Call (443) 364-3676 or send the form below and one licensed contractor calls you back within one business day.
ADU site prep questions
What does ADU site prep include?
Clearing the footprint, cutting and filling to a level pad a foot or two larger than the foundation, compacting the fill to the building permit standard, shaping a swale so water leaves the lot the way the plan shows, and trenching the sewer, water and electrical runs to the main house or the street.
How much does an ADU pad cost in San Bernardino?
The pad alone runs $3,500 to $10,000 for the rough grade and compaction. Add the utility trenches, often $1,500 to $5,000 depending on how far the sewer has to go, and the excavation side of most ADUs lands between $5,000 and $14,000.
Do I need a compaction report?
Yes, in every valley city, before the foundation inspection. On a flat lot the grader’s report is usually enough. On a hillside the city wants a soils lab testing each lift, and sometimes a geologist review.
Can the ADU share my existing sewer lateral?
Usually, if the lateral is in good shape and sized for both. Many 1950s and 1960s laterals in San Bernardino and Rialto are not, and owners replace the lateral and tie the ADU in during the same trench. The cost guide has the lateral range.
How big can my ADU be?
State law requires cities to allow at least an 800 square foot detached unit on nearly any single family lot, and larger units are allowed under most local codes. The California HCD ADU handbook lays out the floor.