Hard wired fire alarm for 3 storey small HMO

HMO Licensing

Hi All

I have been reviewing my current fire setup for my 3 storey (I think) additional license hmo and would welcome some definitive feedback.

The property comprises of

Downstairs - stairs with small porch + downstairs toilet , kitchen and lounge area all one room with 1 door for entry from porch and 1 door to exit to decking area outside

1st floor 2 bedroom

2nd floor 2 bedrooms and bathroom

My question is first of all is the property 3 storeys as Iv heard the ground floor doenst count as a storey ?

Most importantly do I need a mains powered fire alarm or will a battery powered interlinked one suffice as this is not a mandatory HMO and all bedrooms have windows for escape all be it top floor is higher than 4.5m above ground.

I have read and reread Lacors plus FSO but again come up against differing answers.

Any help would be greatly appreciated.