ADU Foundation Pouring and Slab Construction in San Diego
San Diego is one of the hottest ADU (Accessory Dwelling Unit) markets in California. New state laws have streamlined permitting, and homeowners across the county are adding backyard units for rental income, multigenerational housing, or home offices. Every ADU starts with a foundation — and SD Concrete Pros is the concrete contractor San Diego homeowners trust for ADU slab work.
We pour ADU foundations for detached units, garage conversions, and attached additions. Our work meets all current California Building Code requirements and passes city inspection the first time.
ADU Foundation Types We Build
- Slab-on-Grade: The most common ADU foundation in San Diego. A monolithic concrete slab poured directly on prepared ground, with thickened edges that serve as the footing. Cost-effective and fast. Suitable for flat to gently sloped lots.
- Raised Foundation (Stem Wall): A perimeter concrete foundation wall with a crawl space beneath the floor. Required for sloped lots or when the finished floor must be elevated. More expensive but provides access to plumbing and utilities beneath the unit.
- Post-Tension Slab: A slab reinforced with high-strength steel tendons that are tensioned after the concrete cures. Used on expansive soil or where an engineer specifies. Common in San Diego’s clay-heavy soil areas like Tierrasanta and Rancho Penasquitos.
- Garage Conversion Slab: When converting an existing garage to living space, the existing slab often needs modifications — raising the floor level, adding insulation, leveling, or pouring a new slab over the old one.
San Diego ADU Requirements
ADU construction in San Diego involves specific foundation requirements:
- Permits: The City of San Diego and most county jurisdictions require a building permit for ADU construction, including the foundation.
- Soils Report: A geotechnical investigation is typically required to determine soil bearing capacity, expansive soil potential, and foundation design parameters.
- Engineering: Foundation plans must be stamped by a licensed civil or structural engineer.
- Inspections: City inspectors must approve the excavation, forms, reinforcement, and anchor bolts before the pour.
- Utility Connections: Plumbing rough-in, electrical conduit, and gas line sleeves must be placed in the foundation before pouring.
SD Concrete Pros coordinates with your architect, engineer, plumber, and electrician to ensure all trades are scheduled in the right sequence. We do not just pour concrete — we manage the foundation phase of your ADU project.
Our ADU Foundation Process
- Plan Review: We review your engineered foundation plans and verify dimensions, rebar specifications, anchor bolt placement, and hold-down locations.
- Site Preparation: Excavation to specified depth, removal of organic material, and compaction of native soil or import fill.
- Sub-Base: Compacted Class II base rock installed to engineer-specified thickness. Verified with compaction testing.
- Moisture Barrier: 10-mil Visqueen vapor barrier installed per code, with sealed seams and proper overlap.
- Forming: Precision forming for slab edges, step-downs, thickened edges, and interior footings. All forms verified for level and dimension.
- Plumbing and Electrical: We coordinate with your plumber and electrician for rough-in placement before rebar installation.
- Reinforcement: Rebar installed per engineering plans — typically #4 rebar on 12 to 18-inch centers both ways, with continuous rebar in footings and thickened edges.
- Inspection: City inspector verifies forms, rebar, plumbing, moisture barrier, and anchor bolt placement before we pour.
- Concrete Pour: 3,500 to 4,000 PSI concrete delivered by ready-mix truck. Placed by pump or chute, vibrated, screeded, and finished to specified tolerance.
- Anchor Bolts and Hold-Downs: Set while concrete is fresh, positioned per plan for wall framing attachment.
- Curing: Wet cure or curing compound applied per specification. Slab protected from traffic during the 7-day initial cure period.
ADU Foundation Costs in San Diego
- Slab-on-grade (standard ADU 400-800 sq ft): $8,000 to $18,000
- Raised foundation (stem wall + floor): $15,000 to $30,000
- Post-tension slab: $12,000 to $22,000
- Garage conversion slab work: $3,000 to $8,000
Foundation typically represents 10 to 15 percent of total ADU construction costs. A well-built foundation prevents costly structural problems down the road.
Frequently Asked Questions
How long does an ADU foundation take?
From excavation to pour, most ADU foundations take 5 to 10 working days, depending on size and complexity. Add 7 days for initial cure before framing can begin.
Do I need a soils report?
Almost always in San Diego, yes. The geotechnical report tells your engineer what type of foundation your lot requires. Costs $1,500 to $3,000 and is required before engineering begins.
Can you build an ADU foundation on a slope?
Yes. Sloped lots require stem walls or retaining walls as part of the foundation system. This adds cost but is a common San Diego scenario — we have extensive experience with hillside foundations.
What about Junior ADUs (JADUs)?
JADUs are typically built within existing structures (garage conversions or room partitions) and may require minimal foundation work — usually slab leveling, moisture barrier installation, or thin overlay for floor height adjustment.
Start Your ADU Foundation Project
Call SD Concrete Pros at 619-848-3880 or email info@sdconcretepros.com. Bring your plans — we will review them and give you a detailed foundation bid within 48 hours.