Within any single type, the fee still moves with the size and height of the building, the complexity of the layout, how smooth access is on the day, and the quality of the records you hand over. A well-documented modern block with a responsive site contact is a quick job; a poorly recorded conversion with locked cupboards is not. Volume matters as well: portfolio programmes are priced more keenly per building than one-off instructions. Location moves the fee a little too, mostly through travel time, but far less than scope and size do.
The cheapest number is not automatically the right one, because the legal test is whether the finished assessment is suitable and sufficient, and the responsible person keeps that liability whoever wrote the report. For what a fair fee has to include, and the warning signs of a fee that is too low, read how much should a fire risk assessment cost.