Depends what you mean.
The compartment wall only has to run the full vertical height of the building if it is separating two buildings.
Within a building a compartment walls can subdivide a single storey level but must run from one compartment floor through the full height of the storey to the underside of the compartment floor above.
Or are you asking about the total maximum size of a compartment in an unsprinklered shop, which is 2000 sq m. Provided the total area of the compartment is within this limit then the storeroom need not be separated from the retail area by a compartment wall- but some fire resistance will be needed depending on the size of the store- at least half hour for stores 250-450 sq m, and one hour for larger stores.
So within this you can be a bit more relaxed about what you do in the ceiling void than if you need to provide a compartment wall.