What the structural engineer does
A licensed PE produces:
- Foundation design based on the geotech report's bearing recommendation (conventional footings, pin piles, helical piles, or pier-and-grade-beam).
- Framing plan — joist sizes and spacing, beam sizes, header sizes, lateral bracing.
- Retaining wall design for any wall over 4 ft, or any wall on an ECA parcel.
- Special conditions — large openings, cantilevers, decks, irregular framing.
The structural sheets are stamped by the PE. SDCI requires the stamp on the construction permit submittal.
Cost
- Standard DADU on conventional footings: $2,500–$4,000.
- With piles or significant retaining: $4,000–$6,000.
- Complex / steep-slope: $6,000+.
Verification
PE license via WA DOL — Engineer & Land Surveyor Search. Confirm structural specialty.
When the engineer joins
After schematic design (step 2.2) is signed off and the geotech report (step 1.3) is in. Earlier is wasted iteration; later is expensive rework.
Where this information came from
- SDCI Tip 116B — Establishing a DADU · retrieved April 22, 2026
- Building Connections — Side sewer transfer to SPU · retrieved April 22, 2026
- King County Wastewater Capacity Charge · retrieved April 22, 2026
- L&I Verify a Contractor · retrieved April 22, 2026