BS12056-2 (clause ND.3.5.3) requires a dedicated stack to serve appliances on the lowest two floors when the stack rises over 20 storeys. If there happen to be no appliances connected to the lowest two floors of the vertical section of the stack, then the one stack would be compliant - however consider that appliances may be added in the future and so provision of a second stack for the lowest two-storeys should be considered for future proofing and design development. While a secondary ventilated system might also be used in this scenario, it is not the height of the stack that makes this necessary and inclusion of secondary ventilation would not negate the need for a two storey stack. Likewise, the diameter of the pipe is dependent on drainage load, not the height of the stack.