SR 503 begins concurrent to SR 500 at an intersection with Fourth Plain Boulevard in Orchards. The two highways travel north on the four-lane 117th Avenue through Orchards until SR 500 turns east onto Padden Parkway, while SR 503 continues north into rural Clark County, passing Prairie High School. The roadway bypasses Brush Prairie on its west side and crosses the Chelatchie Prairie Railroad, before it turns northeast onto 122nd Avenue. SR 503 enters Battle Ground as 10th Avenue and intersects the eastern terminus of SR 502 west of Battle Ground High School. The highway crosses the East Fork Lewis River and leaves Battle Ground to pass Lewisville Park, listed on the National Register of Historic Places and located in Lewisville. SR 503 continues heading northeast into a heavily forested region of the Cascades foothills, toward Fargher Lake and Amboy.