We've just started doing the same thing recently!
First thing you need to do is find the area Development plan which is available on the county council website. Read this and familiarise yourself with the conditions applicable to you -i.e open space requirements, zoning of the area, parking, road frontage, water & sewerage.
Then get in touch with the council and request a pre-planning meeting.
At that meeting, bring your proposal and all the information you have and they will be able to guide you as to whether it would be positive or negative. We had ours 3 wks ago, and they were very helpful.
We've now gone to an architect and QS, to get quotes for the application process. These costs should come in under 4K from what we've heard.
That will take us up to the approval/ rejection stage, and after that we can go on as required.
Hope that helps you,
Michelle.