Consider a typical restaurant premises, would you consider the kitchen to be an ancillary part for the purpose of Building Regulations?
 

Before Anthony jumps down my throat, I did post this on another forum for which I have the upmost regard. However, it has fallen apart over the last few years, which is a pity because it is incredibly beneficial to get the views of other professionals in the industry. 
I note some cracking good advice on this forum. 

It is a bit slow on there sadly. Building Regs in England, Wales, Scotland or NI?

It isn't a defined Key Term in ADB, but the Oxford Dictionary suggests "ancillary (to something) providing necessary support to the main work or activities of an organization" A kitchen could be seen as ancillary to the running of a restaurant, nor is it defined as one of the places of special fire hazard in the same document.

You see plenty of restaurants with the kitchen partly or fully open to the seating area and others fully enclosed or with fire shutters, seemingly according to the whim of the Authority Having Jurisdiction for where the premises is built/fitted out.

ADB purpose groups