The series was created by Tony McHale and Mal Young as a spin-off from the established BBC medical drama Casualty, and premiered on 12 January 1999. It is set in the same hospital as Casualty, in the fictional city of Holby, and featured occasional crossovers of characters and plots with both Casualty (which featured dedicated episodes broadcast as Casualty@Holby City) and the show's 2007 police procedural spin-off HolbyBlue. Its first executive producers were Young and Johnathan Young, who were succeeded by Kathleen Hutchison from 2002 to 2004, Richard Stokes from 2004 to 2006, McHale from 2006 to 2010, Belinda Campbell from 2010 to 2011, Johnathan Young from 2011 to 2013, Oliver Kent from 2013 to 2017 and Simon Harper from 2017. Since the show's initial development, it has become a weekly fixture in the schedule. Holby City airs once a week, all year round, and each series now contains 52 episodes.

is casualty and holby city the same hospital?
yes