An 'H-bar' is a road marking, shaped like an elongated 'H', which is placed across a vehicular access or driveway to let people know they should not park in that area. The width of the driveway is marked out from a point where the kerb starts sloping down (usually 1m from the edge of the driveway).
You can apply for a marking if the entrance of your property has a dropped kerb, however not all dropped kerb driveways are eligible.
The cost depends on the length of the marking required and will include a standard £60 assessment fee. More information can be found with the application form. It normally takes 12 to 24 weeks from application to completion of the marking.