a few thoughts on the extension and answers to the questions..
the nice part.. good luck and i hope it's what you want
the not so nice part,
try and get some advice from the person doing the drawing's
1. i have no idea of the cost per sq foot
2. if you specify what you want ie' electric and plumbing the builder could organize it for you.all prices should inculde windows and doors anyways
3. most builders don't do tiling or painting and recommendations from firnds and family may be a good idea here
4. you will probably find you are missing loads but not to worry about that as a ny good builder will give his opinions and advice wether you want it or not
5.some builders will source a kitchen but i my experience you will end up going to a kitcehn company seperately anyways to get the kitchen
6. ask to see some of his work and then ask the people he did it for.if he's good then he shouldn't have a problem with that
7.did any neighbours ,friend,family have an extension recently... ask them this will also help with 6 above and the tilers.kitchen etc ... you can always look for one in the book or ask the person who did the drawings but make sure you get a few out to price the job
ask all the questions you want.that's what this place is for