Structural engineering · Seattle, WA

Structural Engineering Services in Seattle, WA

Connect with engineering firms serving Seattle and the wider Puget Sound region. These firms handle everything from load-bearing wall removals and home additions to backyard DADUs, seismic retrofits and more.

Also searched as: residential structural engineer Seattle WA · stamped structural calculations Seattle · Seattle ADU structural engineer · Seattle seismic retrofit engineer · structural engineer near me Seattle

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Local code & loads

Seattle building code & ASCE 7 design loads

Seattle enforces the 2021 Seattle Building Code, based on the 2021 IBC. Stamped calculations submitted for permit reference the design parameters below.

Values shown for reference only — project-specific values from the ASCE 7 Hazard Tool should be used for engineering design at the actual project address.

ASCE 7 design parameters

Default values used for most residential and light-commercial projects in Seattle. Site Class and seismic spectral values may shift based on geotechnical investigation.

Building code	2021 Seattle Building Code, based on the 2021 IBC
ASCE 7 edition	ASCE 7-22
Seismic Sₛ	1.57g
Seismic S₁	0.66g
Site class	Default
SᴅS	1.19g
Sᴅ₁	0.92g
Seismic Design Category	D
Ultimate wind speed	98 mph
Ground snow load	20 psf (Puget Sound lowland; per SEAW snow load guidance)

Risk Category II assumed for typical residential and light commercial use — confirm with your engineer if the occupancy or use may require a higher category.

Seattle is a high-seismic city. The shallow Seattle Fault crosses the city east to west, and the offshore Cascadia Subduction Zone adds the threat of a long, strong megathrust event. Soils change quickly over short distances, from dense glacial till to soft fill and river alluvium near Elliott Bay and the Duwamish, so liquefaction often drives foundation design in low-lying and waterfront areas. The numbers below are regional placeholders only. Always pull site-specific values from the ASCE 7 Hazard Tool (https://ascehazardtool.org) for the actual project address.

Common projects engineers design in Seattle

The work engineers handle frequently in Seattle and the surrounding King County.

Load-bearing wall removals and open-concept remodels
Detached accessory dwelling units (DADUs) and basement ADUs
Room additions and second-story additions
Seismic retrofit of older wood-frame and unreinforced masonry homes
Foundation work on fill, soft soils, and sloped lots
Deck, balcony, and rooftop deck structures
Rooftop solar array structural verification
Retaining walls on Seattle's many hillside lots

What's different about Seattle

Local conditions that affect structural design

City-specific factors that show up in many Seattle calculations — and the things plan reviewers look for.

Seattle sits in one of the more active seismic settings in the country. Between the shallow Seattle Fault and the Cascadia Subduction Zone offshore, lateral design and a clear load path matter for almost any structural project, and older homes often need bracing or hold-down upgrades during a remodel.

Fill and soft alluvial soils near Elliott Bay, the Duwamish, Harbor Island, and the SODO flats carry real liquefaction risk. A geotechnical report is commonly needed in these areas, and standard spread footings are not always enough.

The city dropped its owner-occupancy requirement for accessory units, and many lots can now add both an attached ADU and a detached DADU. That change has made backyard cottages one of the most common reasons homeowners come looking for stamped structural calculations.

Seattle has a large stock of older houses and roughly a thousand unreinforced masonry buildings. Voluntary seismic retrofit work is steady, and a mandatory URM program has been in the works, so retrofit and load-path upgrades come up often.

Permits

Where to submit your Seattle permit

Stamped calculations from clearspan are submitted directly to the Seattle Department of Construction and Inspections (SDCI). Most residential structural submittals are reviewed online.

Building department

Seattle Department of Construction and Inspections (SDCI)

700 5th Ave, Suite 2000, Seattle, WA 98104

(206) 684-8600

Visit the building department →

Submittal notes

SDCI handles permits inside Seattle city limits. Properties in unincorporated King County go through King County's Department of Local Services, Permitting Division instead, which is a separate jurisdiction. Check which agency covers the project address before you submit.

FAQ

Structural engineering in Seattle — FAQs

The questions Seattle contractors and homeowners ask most often.

Do I need an engineer to remove a wall in my Seattle home?+

If the wall carries load, stamped structural calculations from an engineer will help you avoid plan-review comments from SDCI and keep the permit moving.

Does a backyard DADU in Seattle need stamped structural calculations?+

Most detached ADUs go through a full building permit, and the structural framing and foundation usually need to be shown with engineered calculations. An engineer through clearspan can provide the sealed calcs SDCI looks for, and soft or sloped sites may also need a geotechnical report.

How quickly can I get stamped calculations for a Seattle permit?+

Most residential beam and wall-removal calculations are turned around within a few business days through clearspan's process. Timelines depend on scope, and availability of an engineer to review and accept the design through clearspan.

Nearby cities

Structural engineers in other Puget Sound cities

Browse city pages for nearby jurisdictions.

Bakersfield, CA→Citrus Heights, CA→Clovis, CA→Concord, CA→Davis, CA→Elk Grove, CA→

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