We take it you are the certifier so presumably you have classified it as a Class 2 building.It should be possible to achieve the required fire ratings for the external walls of the lower occupancy, but note that there will also need to be fire separation between the lower and upper occupancies, ie. a fire rated ceiling throughout the lower storey plus the required sound control. Also we believe the external walls of both occupancies will need the same treatment, not just the external wall of the lower occupancy. Our Technical Design Guide #02 may be helpful. It is available for download here: https://www.woodsolutions.com.au/publications.Dual occupancy
Answered on 24-09-2020
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