section 5exemption is given buy the council, take less time than full planning, but worth doing if you are thinking about future sale (check around the forum, I'm sure I've explained this in more detail before)
yes to building regs and bank needs and no your arch will employ/suggest an engineer to ensure structure if required (really only, if there is a substantial length/weight of supporting steel etc)
89.. id be thinking about its BER rating - insulation levels, checking for condensation/ ventilation, asking how old the windows are etc (these are all things your arch/ arch tech can inspect for you)