There are a number of fire engineering strategies you could go for Ashley. But you may not need any.
If the atrium is free of combustibles and separated from any room containing fire load by fire resisitance, then it is as good as a protected staircase. It is, in effect, a large protected staircase. And a single protected staircase might be acceptable.
How should you interpret "free of combustibles?" Well, as an enforcer, I wouldn't object to a substantial sideboard or a large vase of dried flowers or anything like that, that you might find in such a location. The enforcers in your area may have different opinions.
The prohibition notice may be in place because the enforcers do not believe that the atrium will remain free of fire load. They cannot do this. If the RP says in the risk assessment that it will remain free of combustibles then that must be taken as fact. What the enforcers should do, if they doubt this, is to issue an alterations notice. Such a notice would make it an offence to introduce any fire load into the atrium, but would allow the fire strategy of the building to be based around a risk free atrium for the means of escape.
And yes I agree with John, even though you don't have 5mm/minute design density, you can still have an effective sprinkler system, provided that the fire load in all the rooms is not higher than might be found in a residential premises. It may be a valuable compensatory feature.
Of course you have to convince your local enforcers - it's not me is it??
Stu
