It starts on the A3005 on a spur that exists to provide access to/from the A4 in the direction of Brentford; that road has taken over the westernmost few yards of the B363. We head roughly east along Spring Grove Road, going under the Piccadilly Line about half way along, and soon reach the A315.
Originally the B363 ended on the A315 in Spring Grove but now multiplexes east along that road for a couple of blocks before regaining its number to head south along St John's Road. This was originally the western end of the A3004. A short distance later the B363 reaches the A310 in Isleworth, where it ends.