Building control are right to insist on new fire doors which must be made to the FD20 standards laid down in the building regulations. It should be remembered that the door seals must be made of intumescent material which expands when hot and the door hinges must be steel. The need for the fire door protected hallway is as laid down in Approved Document B.
Building Control may be insisting on a sprinkler system for a number of reasons but the usual reason is that the ground floor kitchen / dining room is considered open plan.
If this is the case a sprinkler system must be installed and the first floor windows will become the escape route. Your structural engineer will be able to advise further on this.
With the further kitchen extension it should be borne in mind that permitted development rights can only be used once so the new kitchen may require planning permission when the time comes for it to be built.
