First time posting, so bare with me.
Came accross this question looking for somthing totally differant on google and thought I would put my two pence in.
I am an Engineer, often I would complete planning applications etc.
I would advise if you want something special, particularly to the rear ext. then go with an Architect, but be sure you pick the right one. There is plenty of draughtsmen out there who call them selfs architect. they generally do a good job, but a trained architect, especially the better ones are well worth the money in the long term , generally an architect designed house or extension has a wow factor.
dont be afraid to interview the architect, ask for examples of previous work, photos, if you can visit or call to a previous job, he should have one on site that you could visit with him, check if he is RIAI registered,
http://www.riai.ie/?id=5761
it a more expensive route but in my opinion well worth it, you can also have the architect prepare the planning drawings and then search around for an engineer to design and supervise the build, but be sure to pick one who understands the architects design concept and is confident he can acheive it.
if you only want a standard ext. know exactly want you need and want then a engineer,draughtsman is fine, if not you should be explaining to the architect how you use the house, what ou intend to use the new space for and let them acheive a good design.
One note of caution, when an architect is given a brief like this not only can it be expensive but you sometimes have to be brave and acept there advise, especially if you are not the best at reading drawings. they will be working to an overall vision that we often do not appricate untill you see the final building.
best of luck