There have been a number of changes and additions to the design of the castle throughout the books/films which has lead to a number of different versions of the castle being recreated and filmed.

these are quite noticable in this compliation of hogwarts external shots:

http://www.youtube.com/watch?v=DZQrDG51n_o&feature=related

Also, at the bottom of the following site, there are a list of the changes that were made throughout the films:

http://harrypotter.wikia.com/wiki/Hogwarts_Castle

The current groundwork next to the Great Hall is for an Entrance Antichamber which appears in books 1 + 2 and could be considered "original" hogwarts. However this was swapped out for an entrance courtyard in book 3 and has remained since.

Could be something to think about before continuing with the antichamber.