Seismic Requirements for Horizontal Building Additions

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Hi Folks,

I just wanted to make sure that if my logic is correct or not.I am working on a horizontal building addition to a one story wood framed building. I am planning on analyzing the addition laterally by considering the base shear based on the total load weight of the building, which includes the existing (old) weight and the addition, then distribute the force to the wood shear walls based on tributary areas.
I would like to get some input from an experienced structural engineer.

Thank you for your time and energy.