For a custom home in Orange County, expect a post-tension slab to run somewhere between $30 and $55 per square foot of footprint. A 5,000 sf home with a footprint of about 3,500 sf typically lands between $105,000 and $190,000 for the foundation system alone — slab, tendons, stressing, edge beams, vapor barrier, and the inspection record.
That's a wide range. Five things move it:
1. Soils
The geotech report is where the number gets set. Expansive clays — common in parts of Coto de Caza, Ladera Ranch, and the inland canyons — push you toward thicker edge beams, deeper footings, and sometimes a stiffer mat. Sandy coastal lots are friendlier. A typical OC geotech report adds two to five thousand dollars of foundation cost relative to a stable inland site.
2. Grade
Flat lots pour fast. Hillside lots need stepped footings, retaining at the high side, and drainage on the low side. Every step in the foundation adds formwork, rebar, and concrete — and the steeper the grade, the more shoring you're pricing in before the first pour.
3. Post-tension vs. conventional reinforcement
Post-tension uses high-strength steel tendons that get stressed *after* the slab cures, putting the concrete under compression so it resists cracking from soil movement. It's typically $4 to $8 per square foot more than a conventional rebar slab — but on expansive clays, it pays back in twenty years of crack-free finish floors. On stable sandy soils, a conventional slab is often fine.
4. Edge beams and turn-downs
Modern architectural designs love cantilever overhangs, large rear-yard openings, and pool decks tied to the structure. Every one of those features adds engineered edge beams to the foundation. Expect $5,000 to $25,000 of premium depending on how dramatic the overhangs get.
5. Site access
Tight coastal lots with neighbor easements add pump-truck cost (you can't reverse a mixer up a long driveway), formwork staging in offsite yards, and sometimes night pours. Add 8% to 15% on a constrained site.
What's *not* included in that range
A few things people miss when comparing bids:
- Permit and plan-check fees — paid to the AHJ separately, typically $3,000 to $8,000 for a custom-home foundation permit in OC cities. - Soils testing — the geotech report itself, $4,000 to $9,000. - Structural engineering — the engineer of record's stamp on the foundation drawings, usually billed by the architect's office or as a separate $6,000 to $15,000 line. - Retaining walls beyond the building envelope — if you need landscape retaining outside the foundation footprint, that's a separate scope.
When post-tension is the right call
Post-tension makes sense when:
- The geotech report flags expansive clay (PI > 20 or so). - The architect wants a large open-plan slab with no interior bearing walls. - The finish floor is anything that hates cracks — large-format porcelain, marble, polished concrete. - The project budget can absorb the four-to-eight-dollar premium for a twenty-year payoff.
Where it doesn't: small footprints, sandy coastal lots, and projects where the rest of the framing schedule has structural redundancy that makes slab cracking a cosmetic-only concern.
How Skyrise quotes a foundation
We bid after the soils report. We bid line-by-line — slab, edge beams, retaining, drainage, waterproofing, mix design, stressing — so you can compare us against any other contractor on the same set of numbers. And we'll tell you when post-tension isn't the right answer for your lot, even though it's the more expensive line.