Garratt, 54, has been on ECB's reserve list since 2003 and replaces Palmer who officiated for 28 seasons.
"Steve's promotion is thoroughly deserved and we are delighted to welcome him to our full list," said Chris Kelly, manager of the ECB's umpires and match operations. "He has developed into a reliable and effective cricket official during the last few years and has earned this promotion following good performances at first-class level. Our congratulations go to him."
The remaining members of the ECB reserve list have been retained with three additions: Andy Hicks from Worcestershire, Stephen Gale from Shropshire and Steve Malone, the former Hampshire, Essex and Glamorgan bowler.
The ECB has 24 full-time umpires on its full list and operates with six to nine umpires as reserves.
Full List Rob Bailey, Neil Bainton, Graham Burgess, Nigel Cowley, Barry Dudleston, Jeff Evans, Steve Garratt, Ian Gould, Patsy Harris, Peter Hartley, John Holder, Vanburn Holder, Richard Illingworth, Trevor Jesty, Allan Jones, Richard Kettleborough, Barrie Leadbeater, Nigel Llong, Jeremy Lloyds, Neil Mallender, Tim Robinson, George Sharp, John Steele, Peter Willey. (This excludes Mark Benson who is a member of the ICC elite panel and will be used by the ECB in domestic matches when available).
Reserve List Martin Bodenham, Keith Coburn, Nick Cook, Stephen Gale, Michael Gough, Andy Hicks, Steve Malone, David Millns, Terry Urben.