Thank you again for all of these very helpful responses!
Will definitely take account of my neighbours with whom we have very good relations; definitely would not want to jeopardise that. Also, my teens are relatively quiet ... . But we will definitely consider sound-proofing and the impact any noise or disturbance will have on those around us. That would be a top priority.
Rollercoaster yes I like the idea of brick but think we may run in to "permanent structure" territory in terms of planning ... will investigate ... . But I think that explains the thousands of wooden structures about!
Noted about proper builder, thank you Angliski
Good luck with your project LaurieSchafferIsAllBitterNow
Love your garden room (like a mini loft or coach-house) Chumleymouse!
That sounds great chukwe and the cost is justified if you use it 12 hrs plus a day! Mmmm, teens being teens, I can imagine forking out for this and it being "too far" to walk down the garden in the dark!
Maybe not if we make it attractive enough. I think it is important that it will add value to the house overall though.
Thank you to pp below who said consider access and if it all has to be carried through house - have noted that - thank you! Sorry can't find your post - rushing - but very helpful! 
pegasussy about the slugs, I loathe them too so understand your concern. Ours do come in through kitchen door (that leads on to terrace) but I don't know how as there are no gaps that I can see. I put down a very rough door mat (stiff rust brown coir type thing??) and, judging from their silvery deposits early the next morning, that seems to make them turn around and go in the other direction!