Which of the following is excluded when calculating the storey of a building?

The correct answer is that basements used as storage garages when separated by a 2-hour assembly are excluded when calculating the storey of a building. This is because the building code often provides specific provisions for basements, particularly concerning their use and separation. When a basement is designated for storage or garage use and is appropriately separated by a fire-resistance rating (like a 2-hour assembly), it is generally considered separate from the building's storey count. This differentiation is significant, as it recognizes certain uses and safety measures without impacting the overall storey calculation.

The other options involve different scenarios where spaces may contribute to the storey count or have specific guidelines based on their characteristics. Service rooms on the top floor typically count toward the total number of storeys, as they are part of the building's functional area. Mezzanines that are open and do not exceed 40% of the floor space can also count as a floor if they meet certain criteria outlined in building codes, thus impacting storey calculations. Elevator shafts are integral parts of a building's vertical circulation and do not typically affect the count of storeys as they are not habitable spaces but rather necessary components for movement within the building.