Foundation Types in California: Raised, Slab, Pier, and Post-Tension
Which foundation belongs under which lot — and why it matters for your remodel.
California's geology and seismic risk make foundation selection a non-trivial engineering decision. This article covers the four common types, the soils that drive the choice, and the cost and constructability implications for additions and ADUs.
Raised perimeter (continuous footing)
Concrete stem wall, 18–36 inches above grade, with crawlspace below. Dominant in pre-1960 California housing. Pros: easy plumbing/electrical access, retrofittable, good for hillside. Cons: rodent/pest entry, vapor barrier required, energy-loss path. Typical addition cost: $40–$70/sf footprint.
Slab-on-grade
Concrete slab poured directly on prepared subgrade. Dominant since 1970. Pros: cheap, fast, no crawlspace pests. Cons: post-construction plumbing changes are demolition events; cold; cracks reveal soil movement. Typical addition cost: $25–$45/sf footprint on flat lots; +$15/sf if grading required.
Post-tension slab
Slab-on-grade with cable tendons stressed after pour. Standard on expansive clay (San Fernando Valley, Tri-Valley, parts of East Bay). Pros: spans soil movement, large slab without joints. Cons: can't core-drill after the fact without engineering review (cutting a tendon is dangerous). Typical addition cost: $35–$55/sf.
Drilled pier / caisson
Concrete-filled holes 12–24 inches in diameter, drilled to bedrock or competent soil (often 20–60 feet deep). Standard on hillside lots and any project requiring deep foundation. Pros: bypasses surface soil problems. Cons: expensive ($25,000–$80,000 for a typical ADU foundation), specialty contractors only. Add a grade beam to tie piers together.
Underpinning existing foundations
When adding a second story, basement, or adjacent addition, the existing foundation may need underpinning — concrete piers driven beneath the existing footing to transfer new load. Cost: $300–$1,200 per linear foot. Always engineered.
Authoritative sources
- California Building Code Chapter 18 (Soils and Foundations) — California Building Standards Commission
- Post-Tensioning Institute — PTI
- ASFE / Geoprofessional Business Association — GBA
More from the library
- Residential Structural Engineering 101 for California Owners — Load paths, shear walls, and why your remodel needs an engineer's stamp.
- California Title 24 Energy Compliance: A Plain-English Guide — Why your remodel needs a HERS rater and what the energy compliance form really says.
- Seismic Retrofit Basics for California Single-Family Homes — Cripple walls, sill bolting, and the Earthquake Brace + Bolt grant program.