Concrete Foundations in Seattle
A foundation is the single most consequential pour on any structure: get it right and the building lasts, get it wrong and every later trade pays the price. We pour residential and commercial foundations across the Seattle area — new construction, ADUs, additions, and structural retrofits — working from stamped engineering plans and coordinating with the engineer of record and SDCI inspectors through each phase.
Seattle’s soils vary dramatically by neighborhood: glacial till that supports spread footings well across much of the city, fill soils that require deeper systems in SoDo and Interbay, and steep hillside lots that may need engineered grade beams or drilled piers. The right foundation system is determined by the geotechnical report and structural engineer, not by guesswork; we build to what the engineering calls for and document each pour for inspection.
Foundation construction process
A foundation pour follows a sequence that doesn’t tolerate shortcuts: surveyed layout from the stamped plans, excavation to the bearing stratum identified in the geotech report, compacted base prep, formwork built to engineered dimensions, rebar placement inspected against the structural drawings, and finally the placement and finishing of concrete. We schedule pours around weather windows — Pacific Northwest rain affects both slump and curing — and coordinate inspection holds with SDCI so each phase is signed off before the next is covered. Where the engineer specifies pumped concrete or specialty mixes (high early strength, fiber-reinforced, low-shrinkage), we order from the appropriate Cadman, Salmon Bay, or Stoneway plant based on truck-time-to-site.
Materials and reinforcement
Most Seattle-area residential foundations use 3,000–4,000 psi mix designs with type I/II cement, #4 or #5 rebar tied to plan, 10-mil vapor barrier under slab-on-grade pours, and crushed-rock base over compacted native soil. Stem walls and grade beams take heavier reinforcement schedules — horizontal and vertical bars on engineered spacing, often with corner ties and dowel connections to the footing. Where the geotech calls for it, we add capillary breaks, sub-slab drainage, or waterproof membranes on basement walls. Mix design and reinforcement schedule are engineering decisions, not contractor preferences; we build to what the stamped plans call for and document each step for inspection.
Typical foundation projects
The most common projects we pour are slab-on-grade foundations for ADUs and single-story additions, continuous footings with stem walls for two-story wood-frame construction, grade beams on drilled piers for hillside lots where standard footings can’t reach competent soil, and underpinning or supplemental footings for additions that need to tie into an existing foundation. We also pour commercial slabs and footings for small-to-mid-size buildings — restaurants, retail, multifamily — where the schedule and inspection coordination differs from residential work. Foundation repair (cracks, settlement, water intrusion) is handled on our basement repair and concrete repair service pages.
Seattle soil and seismic considerations
Most of Seattle sits on glacial till - a dense, stable mix of sand, gravel, and clay that generally supports spread footings well. The exceptions matter: sites with fill soil (common in much of SoDo, Interbay, and areas near the waterfront), steep hillside lots where the bearing stratum is several feet down, and older properties where previous grading left unpredictable subgrade. Seattle's seismic zone (Zone 2B/UBC) also requires that foundation reinforcement meet IBC seismic provisions. We build to the stamped engineering plans and coordinate with the engineer of record and SDCI inspectors through each pour.
Foundation cost in Seattle
| Foundation type | Typical cost | Notes |
|---|---|---|
| Slab-on-grade (per sq ft) | $6–$14 | Includes base prep, vapor barrier, rebar, pour |
| Continuous footing (per linear ft) | $40–$90 | For wood-frame stem wall construction |
| Stem wall + footing (per linear ft) | $80–$150 | Full perimeter with crawlspace |
| Foundation repair / underpinning | $5,000–$20,000+ | Depends on scope and pier count |
All estimates require an on-site visit. Permit fees, engineering, and geotech reports are separate from construction costs.
Frequently asked questions
What types of foundations do you build?
Spread footings, continuous footings, slab-on-grade, stem walls, grade beams, and pier-and-beam for additions and ADUs.
Do you handle inspections and engineering?
Yes — we coordinate with the engineer of record and Seattle SDCI inspectors. We don't stamp engineering, but we build to the stamped plans.
How long does a foundation pour take?
Excavation through pour is typically 1–3 weeks for a residential addition or ADU, depending on access, soil conditions, and weather.
What's the right foundation type for Seattle soil?
Most of Seattle is glacial till, which holds well for spread footings. Fill soils, steep slopes, and waterfront properties often need engineered solutions.
Do you do foundation underpinning or repair?
Yes — for settling foundations we work with a structural engineer to design and install underpinning, helical piers, or wall reinforcement.
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