Customers to the bar will include aides, servants and protection officers, and they will be able to relax with a drink in a cosy lounge in the Piper's Hall.
Previously thirsty workers had to go to the village of Ballater ten miles away, but now they will only have to travel just 50 yards from the Highland castle on the remote Aberdeenshire estate.
"Piper's Hall is a great setting, right next to the river. I guess you could say it'll be one of the most exclusive bars in Britain," News of the World quoted a senior aide to the Queen as saying.
"The Queen and the Duke of Edinburgh will visit the new bar without a doubt. The staff fancy dress party has been held in the Piper's Hall and she always popped in to pick the winner.
"She's partial to a gin and Dubonnet while the Duke likes a single malt," the aide said.
Applications for planning permission and an alcohol licence have been submitted.
The bar will open seven nights a week from Aug until Mar but the hall will revert to a tourist cafe and shop when the estate is open to the public.