It extended 10.4 miles (16.7 km) from Pillar Rock northwest to SR 4 in Rosburg.

SR 403 went west and passed the communities of Brookefield and Altoona while paralleling the Columbia River's shoreline.

The highway then went northwest and then curved westward before reverting to a more northern route to Rosburg.

[7] SSH 12C became SR 403 in 1964 during the 1964 highway renumbering, in which the Washington State Department of Transportation (WSDOT) replaced the previous system of Primary and Secondary Highways with a new system called State Routes, which is still in use today.

[4] The decommissioning took effect on April 1, 1992, with the state government continuing to maintain the Grays River bridge.