There have been cases cited in these pages of new Fire Safety Inspection companies demanding an expensive upgrade of buildings, especially new ones that they have recently taken over.
This can be for a number of reasons, most of them valid.
One would be that the fire safety measures in the building were out of date due to changes in legislation.
Another would be that they want to get everything right before they take over the contract.
A third would be that they want to provide a Rolls Royce solution to a Toyota Corolla problem because they can make more money that way.
The authorities are unlikely to allow you any leeway, regardless of the age of the building. They are simply concerned with fire safety, irrespective of the cost.
However, you can ask the inspectors to itemise what in the building is no longer compliant and under which aspect of changes in the law.
It may be that requiring them to justify each of their charges rather than impose a one-size-fits-all solution could save you a lot.