"Would you expect him to be able to give you some advice about layout / what would be allowed / planning permission process etc and / or should I be armed with lots of questions."
Yes to all that and I agree with much of the good advice given above.
I always ask lots and lots (and even more lots) of questions to really get behind the (potential) client's reasons, needs, aspirations, and so on as well as more mundane items such as budget, timescales, any constraints such as ownership, access, neighbour issues,etc.
Often the client's request for a specific building project - say as in your case an extension of a particular size and shape may, after much talking and exploring of the clients actual needs rather than their solution result in an entirely different project or even, and this has happened a few times, no project at all.
I have talked myself out of more than one commission when I genuinely believed it was in their best interest and instead recommended a house move.
Don't forget to ask about fees, references, any other consultants they may require and statutory charges (planning, building regulations etc).
Oh - and check that the Architect is actually an Architect. The title is protected by law and this is regulated. Sadly the function is notprotected and anyone can claim to be able to undertake design work, so be sure you check.
In any event, I wish you well.