Retrospective building work .

HMO Licensing

Good afternoon,
I have an HMO which has been licenced for the last 3 year.(it has been an HMO for many years owned by others) I have had a recent inspection and the council (Wirral) have issued an improvement notice with regard to fire safety. The improvement includes emergency lighting and a mains wired alarm system which I am fine with but the have stipulated other works. The floor boads have to be taken up and insulation fitted between cieling and floor,
bathrooms to have half hour fire resisting structure and self closinf fire doors if an acrylic bath or shower tray is fitted and an electric shower is present.
Also the understair soffits are to be fitted with fire check board.
I was under the impression that this type of work was for new build or property undergoing major refurbishment and coming under the scope of current Building Reg's not for existing properties in current use, as the costs involved do not make the property financially viable.
Have you any information on the legality of the Council enforcing such changes
Best Regards, Artro