It starts at a roundabout on the B455 in Castle Bar Park and heads north, soon crossing the River Brent. The next turn right (at traffic lights) is Perivale Lane, which was the original route of the B456; however, for reasons which will soon become apparent it has been rerouted further north to a GSJ on the A40.
The mainline of the B456 crosses the A40 and bears right to join it; both slip roads are unusually long (the eastbound onslip is even two-way for most of its length as if forms a service road) and the A40 is eventually met at the eastern end of Perivale Lane (which no longer has a junction with the A40), by the Hoover Building. The next few hundred yards of the original route of the B456 were taken over by the A40 when that was built later in the 1920s (although it was originally the A403). The B456 soon reappears, although this junction is a LILO and therefore does not provide access to the westbound A40. The road goes under the Central Line before ending on the A4005 to the north of the Hanger Lane Gyratory.