We are in a converted house with two flats (one upstairs, one downstairs) and a small shared communal hallway (less than 2m²). The FRA indicated that we need to install mains powered fire alarm, Grade D Category LD2 with a connected heat detector in each flat.However, I’m struggling to find clear guidance that a communal alarm is required at all in a small two-flat conversion, as I understand these are usually designed as “stay put” buildings with alarms only inside the flats. Can anyone clarify whether a communal alarm system is legally required? We will be installing fire doors if that helps.

In addition, if it is required, does this mean the alarms inside the flats must be mains powered as well, or can battery powered alarms be used if they are wireless interlinked?

Any guidance would be appreciated.

Usually unless the Building was converted in line with post 1991 Building Regulations Guidance it is unlikely (but not impossible) to meet the required standard for stay put - however in small conversions like this with minimal common space if there is at least 30 minutes notional fire separation between flats there is potential for not requiring a common system - I have dealt with cases like this.

If you did have a common system it would have to be Grade D (mains with battery back up)even if wirelessly linked. Grade F these days is only really for individual existing single dwellings provision by householders or landlords where not part of a rewire upgrade.

Thanks. I already have a smoke and heat in the flat(battery powered only). The heat alarm is in the kitchen.   But does the new heat interlinked alarm need to go in the kitchen also? I thought maybe it can just go near the hallway, close to the entrance? 

If it's a heat alarm as part of a common  system going in your flat the hallway close to the entrance is the usual location.