Geotech & Drainage for New Construction in California

When a geotech report is required

Virtually every California hillside parcel, every parcel inside a Seismic Hazard Zone (liquefaction or earthquake-induced landslide), every parcel with known expansive soils or prior fill, and most projects with basements, pools, or significant retaining will require a current geotechnical investigation. Many flat-lot custom homes also require one — local jurisdictions, lenders, and structural engineers commonly require it even when the city does not strictly mandate one.

Soils investigation — what is in the report

• Site reconnaissance

  Geologist / engineer walks the lot and reviews published geologic and seismic maps.

• Subsurface exploration

  Borings, test pits, or CPT (cone penetration testing) to characterize subsurface conditions.

• Lab testing

  Soil classification, expansion index, shear strength, consolidation, and moisture content as appropriate.

• Engineering analysis

  Bearing capacity, settlement, lateral earth pressures, slope-stability factor of safety, liquefaction potential, seismic site class.

• Recommendations

  Foundation type, allowable bearing pressures, retaining-wall lateral loads, drainage and surface-water recommendations, fill placement protocols, special inspection requirements.

Expansive soils

Expansive clays — common across much of LA and the Bay Area — swell when wet and shrink when dry, generating substantial uplift and lateral pressures on foundations and slabs. The geotech report quantifies expansion index and drives foundation design (post-tensioned slab, drilled piers with structural slab, deepened footings) and site grading and drainage strategy. Ignoring expansive soils on a flat lot is one of the most common preventable failure modes in California residential construction.

Slope stability

On sloped parcels, the geotech analyzes static and pseudo-static (seismic) slope-stability factors of safety. The analysis can determine whether the lot is buildable as proposed, requires substantial cut/fill/retaining to stabilize, or requires deep foundations bypassing unstable material. Many California hillside cities require minimum factors of safety (commonly 1.5 static and 1.1 pseudo-static), and failing to meet them is a project-stopper, not a comment to address.

Seismic considerations and Seismic Hazard Zones

The California Geological Survey publishes Seismic Hazard Zone maps identifying liquefaction and earthquake-induced landslide zones, and Alquist-Priolo Earthquake Fault Zone maps identifying surface-rupture-prone fault zones. Parcels inside these zones require additional geotechnical analysis and may have restrictions on habitable structures. The structural design responds with seismic site class, ground motion parameters, and lateral system selection.

Foundation recommendations

The geotech recommendation drives the structural foundation design. Common California foundation types:

• Conventional slab-on-grade

  Flat lots with non-expansive soils and adequate bearing — the cheapest path.

• Post-tensioned slab

  Common where expansive soils are present and the structure can tolerate a stiffened slab.

• Raised foundation with footings and stem walls

  Older convention; still common on remodels and some custom homes.

• Drilled caissons with grade beams

  Standard on hillside lots — drilled to competent bearing, tied with grade beams.

• Driven piles

  Rarer in residential; appears on certain liquefiable or deep-soft-soil sites.

• Basement walls / retaining foundations

  Where the foundation also retains earth (basements, hillside cuts).

Retaining walls and deep foundations

Retaining walls — cantilever, restrained, soldier-pile-and-lagging, or shoring — are engineered against active, passive, surcharge, and seismic loads using the geotech's recommended lateral earth pressures. Hillside foundations and deep retaining are coordinated with the civil drainage design from the start, because most retaining failures are drainage failures in disguise.

Drainage plans — surface, subsurface, and discharge

Drainage on a California new-construction project covers three layers: surface drainage (roof, swales, area drains, ribbon gutters), subsurface drainage (French drains behind retaining walls, foundation drains around basements), and discharge routing to an approved outfall — gutter, public storm drain, or engineered on-site infiltration. The civil engineer produces the drainage plan; the geotech informs the subsurface and retaining drains; the structural design coordinates with both.

Stormwater, SWPPP, and BMPs

California construction sites that disturb one acre or more require coverage under the State Water Resources Control Board's Construction General Permit, with a Stormwater Pollution Prevention Plan (SWPPP) and Best Management Practices (BMPs) implemented during construction. Smaller sites still typically require erosion-control plans and city stormwater approvals. Plans are prepared by qualified SWPPP developers and implemented through construction by qualified SWPPP practitioners.

Erosion control and grading plans

Grading plans show cut/fill quantities, finished grades, slope ratios, and earthwork sequencing, and are submitted with the civil package. Erosion-control plans cover construction-phase BMPs — silt fence, fiber rolls, stabilized construction entrances, and protected drain inlets. Most California jurisdictions require both before grading or demo permits issue.

Hillside, coastal, and fire-area considerations

Hillside parcels combine slope-stability, drainage, and fire-access requirements. Coastal-bluff parcels add bluff-stability analysis and Coastal Commission appeal jurisdiction. Fire-area parcels add WUI envelope and defensible-space requirements. Each overlay layers onto the geotech and civil package — and each is best characterized in the feasibility note rather than discovered at plan-check.

How geotech affects cost, timeline, and design

A geotech recommendation can move a project's foundation cost by 100–300% (slab vs caissons vs deep basement walls), shift the construction schedule by months, and reshape the architectural plan when slope-stability or buildable-area constraints limit the footprint. Geotech is not an end-of-design checkbox — it is a feasibility-stage input.

Frequently asked questions

Do I need a geotech report?

Virtually every California hillside parcel, every parcel inside a Seismic Hazard Zone, every parcel with expansive soils or prior fill, and most projects with basements, pools, or significant retaining require one. Lenders and structural engineers commonly require one even when the city does not. We confirm at feasibility.

How long does a geotech report take?

Typically 4–10 weeks from scope to final report, depending on access, drilling availability, and lab turnaround. Hillside parcels with difficult access or extensive boring requirements sit at the longer end.

What does a geotech investigation cost?

A typical residential geotech investigation in California costs in the low-to-mid five figures, with hillside or extensive-boring scopes pricing higher. The cost is small compared to the foundation design it informs.

Will the geotech tell us what foundation to use?

Yes — the report includes a foundation recommendation (slab, post-tensioned slab, drilled caissons with grade beams, or other), along with allowable bearing pressures, lateral earth pressures for retaining design, and drainage recommendations.

How does drainage affect my project?

Drainage design is coordinated with the geotech, the foundation, and the architectural site plan from the start. Surface drainage, subsurface drainage, and discharge routing are engineered as one package — and most retaining failures are drainage failures in disguise.

Do I need a SWPPP?

Construction sites that disturb one acre or more require coverage under California's Construction General Permit with a SWPPP. Smaller sites still typically require erosion-control plans and city stormwater approvals. We confirm at feasibility.

Can geotech results stop my project?

Sometimes — extremely poor slope-stability, very deep loose fills, active landslide features, or surface-rupture fault traces can make a project infeasible as proposed. The honest output is that information, early.

When should we order the geotech?

At feasibility — before schematic design closes — on any hillside, expansive-soil, or complex-lot project. On flat-lot infill projects, the order can sometimes wait until early design development, but not later.

Official sources

• California Geological Survey — Seismic Hazard Zones ↗

  California Department of Conservation

  Statewide Seismic Hazard Zone (liquefaction, earthquake-induced landslide) maps.

• California Geological Survey — Alquist-Priolo Earthquake Fault Zones ↗

  California Department of Conservation

  Surface-rupture fault-zone mapping that restricts habitable structures.

• State Water Board — Construction General Permit (SWPPP) ↗

  California State Water Resources Control Board

  Statewide construction stormwater permit and SWPPP requirements.

• LADBS — Grading, Excavation & Hillside ↗

  City of Los Angeles

  City of LA grading, excavation, and hillside ordinance reference.

• LA County Public Works — Geotechnical & Materials Engineering ↗

  Los Angeles County Public Works

  Unincorporated LA County geotechnical review and standards.

• California Building Standards Commission — California Building Code ↗

  California DGS

  CBC requirements for soils investigations, foundations, and retaining walls.

Related pages

• New Construction overview →

  Where soils, drainage, and grading work fits into the full design-build process.

• Hillside Construction →

  Caissons, retaining, drainage, and slope engineering for hillside lots.

• Luxury Homes →

  Premium hillside, coastal, and constrained-lot luxury builds.

• Teardown Rebuild →

  Soils, fill, and drainage on teardown lots with existing-condition surprises.

• Permit Timeline →

  How geotech and civil reviews fit into the permit calendar.