Do I Need an Engineer for a Home Addition in Washington?

Washington cities — Seattle, Spokane, Tacoma, Bellevue — review addition plans under their adopted codes, and the PE seal is the standard requirement for anything structural. The seal must show the engineer's name, Washington PE number, and date, with the engineer in responsible charge. Additions that expand the footprint, add a second story, or open up load-bearing walls are treated as structural work everywhere in Washington. Owner-builders don't escape the requirement: the building department asks for the seal regardless of who drew the plans, and the engineer must verify the existing home's framing and foundation can take the new loads.

The engineer reviews the as-built plans and the addition design, designs the foundation for soft wet soils and seismic forces, and checks seismic and high-wind lateral design with proper tie-in to existing shear walls. The tie-in to the existing house gets special attention — ledger and diaphragm connections, differential settlement between old and new foundations, and load path continuity. Bring the site survey, the geotechnical report, and dimensioned as-builts. Western Washington's seismic zone plus wind exposure means additions need a complete lateral load path.

Use this checklist before engaging an engineer for your addition: • Engineer of record holds an active Washington PE license, verifiable on the state roster • Dimensioned as-built plans of the existing home, including framing • Site survey showing setbacks, easements, and existing structures • Geotechnical report addressing seismic and wind exposure for the lot • New foundation designed for seismic and wind exposure — not just copied from the original house • Connection between the addition and the existing house fully detailed • Existing structure verified for the new loads before construction • Engineer confirms they'll respond to plan-check corrections on sealed sheets