A fire cert is definitly required as pvc king said.
As with most applications, you will be required to submit plans, elevations and sections, both proposed and new. I assume the elevations will change somewhat.. i.e signage etc . the section, as the building is existing, does not have to be terrible detailed. Main thing is heights and a section through any additional structure . i.e extension to existing building, or if your altering floor, ceiling, eaves or ridge heights, all this must be shown, and prob be worded in the advert. Without knowing or seeing the extents of changes, I'm afraid i cant tell you exactly what you need.
To sum up.. a new fire cert is needed, with the internals partitions, if stud etc, will most likely need to be upgraded as a restaurant is deemed high risk. Its safer lodging both proposed and existing plans, elevations and sections so as to cover everything from a council point of view. You may also need to submitt details of signage for the proposed office.. if applicable.
Hope this helps..