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
We coordinate layout, excavation, formwork, and reinforcement before placing concrete to meet site requirements and local standards. Every pour is planned for consistent strength and proper curing.
Materials and reinforcement
Foundations typically use region-appropriate mix designs, rebar, vapor barriers, and optional waterproofing to handle moisture, loads, and long-term settlement.
Typical foundation projects
Common projects include slab-on-grade foundations, footings, stem walls, and foundation repairs for additions and structural upgrades.
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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