If I'm wrong, I need to know. I'm going by local friends, the spaces they've had made recently and also google!
We have a 1930's unextended house, with the living room, dining room and small kitchen. We want to knock the rooms st the back together snd extend out the back and side to make a large kitchen diner. Kind of like everyone I know has done! 
They have all told me 40/45/50 at a push...
My quotes so far are 87 and 110!!
I almost had a heart attack.. surely the exact same extension on the exact same house can't be that different??