Most unusual structures (bridges, green houses, gazebos, water storage tanks, fences over 6" high, sea walls, wind generators, solar panels, etc.) will require building permits and, in most cases, engineering as well.
This includes 2 sets of plans & calculations, wet stamped by a currently licensed WA State engineer, along with other required documents. See Required Plans and Submittal Process.
See also: Bridges, Wind Energy
Other helpful resources:
International Code Council
WA Association of Building Officials