I would advise you NOT to get an engineer...they will draw up something that will be structurally sound alright but they are not trained to take into account the design aspects, eg levels of light within rooms, ergonomics etc.
Get an Architectural Technician who is familiar not only with design and the technical aspects of it but also who has experience drawing up planning applications, prefereably within your local area. They should base the design not only on your requirements but also on the orientation of the site.
I know a girl who got an engineer (her cousin) to draw up an extension for her and she has to leave a light on all day in the sitting room! Even if you pay a bit more than you would pay a basic draftsman it will be worth it. Another tip, listen to the professionals, my boyfriend is in the industry and all too often he makes suggestions to people who stick to their guns and build whatever they have in their heads and then regret it later. It could be something simple like moving a door but it can make a big difference when you are living in it!