Hi could you clarify something please. BS9999 16.6.1 says you should discount a storey exit when calculating available exit width (if there is more than one storey exit). However, 17.3.2 says if your escape stairs have protected lobbies you don't need to discount a stair. If escape exits are at the bottom of the stairwells, and the stairwells have protected lobbies, can we avoid discounting one of the exit widths?

No. Calculations for horizontal evacuation are separate from those for vertical.
Stage 1- assess escape from the room

Stage 2- assess escape from the floor

Stage 3- assess escape from the building

All seperate