Soil Conditions in San Marcos
San Diego's geology varies dramatically from neighborhood to neighborhood. Here's what we see most often.
Granitic Bedrock and Decomposed Granite
San Marcos is largely underlain by granitic bedrock ranging from fresh, hard rock on hillsides to deeply weathered decomposed granite in lower areas. DG soils drain well and have good bearing capacity but may require rock hammering for deep excavations.
Santiago Formation — Twin Oaks Valley
The Twin Oaks Valley area in western San Marcos contains Santiago Formation claystones and sandstones with variable expansion potential. Foundation design must account for the specific expansion characteristics of the formation at each site.
San Marcos Creek Alluvium
The San Marcos Creek corridor contains alluvial soils with variable density and clay content. Some areas have shallow groundwater, particularly during wet seasons, which affects foundation design and construction scheduling.
What We Do in San Marcos
Soils Reports
Geotechnical investigation and foundation recommendations for building permits.
Learn more →Grading Plans
Precise and rough grading design with drainage and earthwork calculations.
Learn more →Compaction Testing
Field density testing and fill certification for construction.
Learn more →Stormwater / LID / WQMP
LID compliance, WQMP preparation, and BMP design.
Learn more →Hydrology Reports
Drainage analysis and flood risk assessment.
Learn more →Subsurface Exploration
Drilling, CPT, and test pit programs.
Learn more →Erosion Control Plans
SWPPP and erosion control for grading permits.
Learn more →Street Improvements
Frontage, curb, gutter, and sidewalk design.
Learn more →Utility Plans
Water, sewer, and storm drain design and coordination.
Learn more →Project Coordination
One point of contact managing structural, MEP, surveying, and specialty consultants.
Learn more →We also coordinate with your structural engineer, architect, MEP consultants, and other project team members to ensure our deliverables integrate with the overall design — one point of contact for your geotechnical and civil engineering scope.
What People Build in San Marcos
Single-Family Home
New homes throughout San Marcos on granitic and Santiago Formation soils.
ADU / Accessory Dwelling Unit
ADU permits have increased significantly in San Marcos — soils reports required for new detached structures.
Student Housing / Multifamily
Multifamily development near CSU San Marcos campus, requiring thorough geotechnical evaluation.
Commercial Development
Commercial and mixed-use projects along San Marcos Boulevard and the Restaurant Row corridor.
Working With City of San Marcos Development Services Department
San Marcos requires geotechnical investigations for new construction and grading permits. Reports are submitted with permit applications and reviewed by city engineering staff.
What Makes This Jurisdiction Distinct
CSU San Marcos campus area — specific plan overlay with unique development standards
Hillside development regulations for properties on slopes over 25%
Santiago Formation expansive soils in Twin Oaks Valley area
Grading permits required for earthwork over 50 cubic yards
Stormwater compliance per San Diego Regional MS4 permit
Submit to Development Services at San Marcos City Hall, 1 Civic Center Drive.
Common Questions — San Marcos
Building in San Marcos?
Tell us about your project and we'll send you a fixed-fee proposal — typically within one business day.