The Hammerhead can start at any safe altitude (typically above 3500′ above ground in the Extra 300L) and the maneuver gains altitude, it doesn’t lose altitude. You will gain anywhere from 1,000 – 2,000 feet of altitude during the pull-up and then usually recover at or above the altitude you started at if it is a just a single maneuver you are performing (which is likely the case the first time you do it).