The practice has been appointed by developer Station Properties to draw up the mixed-use scheme, which runs from the Royal Mile to Market Street and also includes the Advocate, Roxborough and Warriston Closes.
Morgan McDonnell director Guy Morgan said the project was complicated because of the jumbled nature of the site and the presence of a range of buildings for which listed building consent has had to be obtained.
Morgan said: 'It is difficult to say how many buildings we are working on - it is such a hotchpotch and jumble. There are buildings from the 16th to the 20th century and we are adding some 21st century ones.'
Morgan added that work on the project will begin in the autumn.