How can I calculate ground floor stairwell enclosure exit width of multi-storey

[RichardfromGreystoke]

An office block having two fully protected stairwells of width 1200mm with the consideration of 9 storeys and fully protected lobbies leading to each escape stairs from each storey to each of the two available protected stairwells in theory can take a volume during simultaneous evacuation of 600 individuals each.
This is in accordance with Table 3.2 ADB Vol 2 in terms of width of stairs for basements and simultaneous evacuation of buildings. 1200mm is capacity suitable for 600 persons.
The question I have is what door width does the ground floor stairwell enclosure leading into the escape lobby need to be?
Building Control indicate that this needs to be 1200mm or more I.e., the same or more than the stairwell width so as not to impede or reduce the exit flow from the stairwell ground floor storey exit.
This does make sense as not all those evacuating will reach the ground floor storey exit at the same time and apparently this number of 600 includes calculations of descending rates. All staff are evenly spread across all floors.
However, through fire risk assessment this is being contradicted by the assessor who disagrees with Building Control.
They conclude that the ground floor staircase enclosure door width leading into the lobby and onto the final exit is purely dependant on the number coming down the stairwell. I.e., if 600 do come down the stairwell ? as allowed in table 3.2 ADB Vol 2 then this ground floor storey exit leading to lobby and onward to final exit will need to be 3000mm width to allow and comply with the current guidance within regulations. If 350 people, as potentially in our case, come down the stairwell then this width would need to 1750mm minimum width.
Does anyone have the competence to help me understand if our max capacity of 700 persons divided equally by the two stairwells I.e., 350 persons per stairwell would mean we would have to increase our ground floor storey stairwell enclosure door width to 1750mm? or is it acceptable to have it the same width or more as the current stairwell width of 1200mm?
Would really appreciate any help with this please.

[Seetek]

You don't appear to have discounted a storey exit.

[RichardfromGreystoke]

Hi Appreciate this but this would be one storey exit only as the storey the fire occurs on would need one discounting. The main question is whether the ground floor stairwell exit is dependant on the numbers coming down the stairs or in the case of the Building Regs examining the flow down a stairwell of width 1200mm, which would allow for a door at least 1200mm or more to allow for the flow of those coming down the stairs to be adequate.

[wee brian]

Final exit door should be about the same width as the stairs (measured differently sometimes but thats the principle) never narrower.  it would be bonkers to ask for more - how would the extra people get there??. The only other factor is if there's a group coming from the ground floor (eg not coming down the stairs) that then merge with the people on the stair - then the door needs to be a bit bigger - there's a method for calculating that in ADB