If I do the extension I'll probably build it myself traditionally but I'm going to contact them out of interest....I bet it's not cheap.
Probably a good move. Your're in the trade,you seem clued up,and, most importantly,you give a damn
If i could slide a personal input,it would be attend obsessively,to the insulation aspect.
Do not leave it to the brickies. I am afraid 99.9% of those i have worked with and know,are not capable of doing this task to a quality normal,in say Germany.
Use rigid panel insulation,tight to the inner skin,you can have a 25mm cavity if needed,don't let BC say you cannot...carry the cavity over the insulated ceiling,good blockwork will aid tight boarding internally,picture-frame the dot and dab,to negate cross draughts...go ----.
You can easily heat a decent size kitchen extension,with one 400mm radiator,door shut,suitable ventilation...i'm stopping now...getting all cold-turkey itchy