The scheme, which has just received a government cash injection of £2.7 million, will open up the existing esplanade and create a new ticket office on the terrace inside the castle walls.
News that Historic Scotland had appointed Hoskins to handle the super-sensitive project to design a replacement for the unloved mobile ticket office was first revealed back in June ( Hoskins in secret Edinburgh Castle plans).
Since then the Glasgow-based practice has managed to land planning permission and Scheduled Monument clearance, and work could start on site before the end of the year.
It is believed Historic Scotland had been concerned about the 'temporary' facilities on the key esplanade site for years and was keen to build a new 'welcome centre' close to the old coalyard entrance - the medieval gate to the castle.
Chris Watkins, Historic Scotland's head of major projects, said: 'This investment shows Historic Scotland's commitment to giving visitors the best possible welcome when they arrive at Scotland's premier attraction.
'We also want people to be able to enjoy the spectacular views of the castle when they arrive on the esplanade without having the existing temporary ticket office in the way.
Watkins added: 'This will be achieved by creating a terrace inside the castle with a new ticket office. The new [facility] will make it easier for visitors to buy tickets and cut down queuing times.'
The ticket office is expected to open for the start of the 2008 visitor season.