Location/minimum smoke detectors in flat

Hi, Im looking to install smoke and heat detectors in a flat and need advice to achieve the minimum required protecting, as asked by the owner. 


Obviously I will install a heat detector in the kitchen and smoke in the living room and hall.

The problem is, it's a second floor flat with an internal staircase which is part of the flat and not communal. 


I though I would need another two smoke detectors, one at the top and bottom? Or is this excessive? 


The bottom of the stairs at the front door is just a 1 metre squared area which goes straight up. The top is very close to the hall smoke detector but a door separates the two areas and the area above door is longer than 10% of room height.

It is better to have too many but customer might not want to spend a fortune on more of the battery radio 10 year detectors than required.


I have attached photos for reference.


Any information/advice is much appreciated.


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