The Foundation: Where Your ADU Meets the Earth
The foundation is arguably the most important part of your ADU. Everything else sits on top of it, and mistakes here are extremely expensive to fix later. Let's break down what happens during this critical phase.
Types of ADU Foundations
Slab-on-Grade (Most Common)
A flat concrete slab poured directly on prepared ground. This is the standard for most ADUs in Southern California because:
- It's the most cost-effective foundation type
- Our mild climate doesn't require deep frost footings
- It works well on flat to gently sloping sites
Raised Foundation
The ADU sits on a crawl space, elevated above ground level. Used when:
- The site has drainage challenges
- You need to match the floor height of an existing home (for attached ADUs)
- Plumbing access is needed below the floor
Pier/Post Foundation
Individual concrete piers support the structure. Common for:
- Hillside properties
- Sites with difficult soil conditions
- Minimizing site disturbance
The Foundation Process
Step 1: Excavation (1-2 days)
Trenches are dug for footings — the wider, deeper concrete strips that sit below the slab. Typical ADU footings are 12-18 inches wide and 12-18 inches deep.
Step 2: Formwork (1 day)
Wooden or metal forms are set up to shape the concrete. The forms must be perfectly level and aligned with the approved plans.
Step 3: Rebar Installation (1-2 days)
Steel reinforcing bars (rebar) are placed within the forms according to the structural engineer's specifications. This steel skeleton gives concrete its tensile strength. Typical elements:
- #4 rebar (1/2" diameter) in footings, spaced per engineering
- Welded wire mesh in the slab
- Anchor bolts for attaching the wood framing above
Step 4: Pre-Pour Inspection
The city inspector must verify the rebar placement, form dimensions, and anchor bolt locations before any concrete is poured. This is a critical hold point — no concrete until the inspector approves.
Step 5: Concrete Pour (1 day)
A concrete truck arrives and the pour begins. The crew works quickly to:
- Fill all forms evenly
- Vibrate the concrete to eliminate air pockets
- Level (screed) the surface
- Finish the surface smooth with bull floats and trowels
- Install anchor bolts while the concrete is still wet
Step 6: Curing (5-7 days)
Concrete needs time to reach its design strength. During curing:
- The slab is kept moist (covered or sprayed)
- No heavy loads are placed on it
- Temperature extremes are avoided
Key Numbers
- Standard slab thickness: 4 inches for ADU slabs
- Concrete strength: 2,500-3,500 PSI (pounds per square inch)
- Rebar coverage: Minimum 3 inches of concrete cover over rebar
- Foundation cost: a property-specific written estimate depending on size and type (slab a property-specific written estimate, pier/grade beam a property-specific written estimate)
Common Issues to Watch For
- Water in the trenches — rain or high water table can delay the pour
- Soil surprises — unexpected rock, old foundations, or poor soil conditions
- Plumbing rough-in — sewer and water lines must be placed BEFORE the slab is poured
- Inspection delays — the inspector must approve before pouring, and their schedule can cause delays
Coming Up Next
Day 7: Framing — watching your ADU take shape as the walls, roof, and structure go up. This is when your project starts looking like a real building.
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Vision ADU can help plan the foundation for your ADU. Get your project started.
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