Hiya, we too are planning an extension to the back of our house. We are using a friends husband to do all the plans etc (he is an architectural technologist, having seen the work he's done to his own house recently, we're more than happy to just go with him and havent interviewed any other architects).
Anyhow, I too was a complete extension virgin, but here's how I understand it / how its working for us:
(1) Architect visits for initial chat (1hr) - we spent alot of time looking at the back of house in terms of where we wanted to extend. Architect came up with a whole new idea we'd never even considered or thought possible just during this initial meeting, so we were super pleased with that! We also had been collecting loads of pictures from house mags and showed him some of the things we liked. He also said we'll do some minor remodelling of the downstairs rooms to make the new space flow / fully usable with the rest of the house.
(2) Architect has now revisited our house and done a full survey - took him 4 hours, measured absolutely everything, used one of those things that stands on legs and you look through like a telescope (to get right levels / measurements etc).
(3) He is now using the survey to draw up plans for us based on our ideas / chats / his extensive measurements.
(4) Once we've all agreed on final design, he takes it all to planning for us (he knows all the local planners at the council so will handle all the planning application etc)
(5) Once planning is through / approved, he then draws up building reg plans - these are the plans that we apparently take to the builders to ask them to quote for the work (although we can take the planning application plans and ask for rough quotes, but they do ultimately need the building reg plans).
(6) Engage builder, build extension, pay builder, sit in new extension and weep at the cost of the remortgage
Ha, sorry, couldn't resist number 6! Seriously though, we are letting our architect guy lead us along the way really, he's the expert, we just tell him what we like / what we want the extra space for and how it'll be used. They're so bloody clever at coming up with brilliant things that me and DH would never even dream of (we're very undesigny people!).
GOod luck, hope all goes well!