The scheme, which will cost at least £1.3 million, is expected to get the nod from Historic Scotland in the next couple of weeks and will spell the end for the unloved mobile ticket office.
The new 'welcome centre' will be built close to the old coalyard entrance - the medieval gate to the castle - and the project could start on site in the autumn.
It is understood Historic Scotland has been concerned about the 'temporary' facilities on the key esplanade site for years.
However exact details about the hush-hush replacement scheme are still being kept firmly under wraps.
Practice chief Gareth Hoskins said: 'It's a very sensitive scheme on the site of the most important monument in Scotland.
'We actually landed the job about a year and half ago after winning an invited competition where we saw off architects including Alan Murray.
Describing the firm's approach to the new centre, Hoskins added: 'The principle of the design is similar to that of Stanton William at the Tower of London.
'It's about how you make queuing up for 200 people interesting and not just about ticky-tacky barriers.
'We are doing a contemporary piece, rather than a pretend old piece. Historical pastiche is not our bag.'