Seismic strengthening or new design  

Kia Ora koutou,

I have a bit of a curly question that i thought might be worth putting out here to see if anyone else has come across the same consideration before.

I have an existing 1950’s in-situ concrete 2 storey building with part height basement that I am tasked with doing the structural upgrade design for. The building is currently at approx. 35%NBS IL2 and the client wants to upgrade to 100%NBS.

The plan for the upgrade is to demolish the existing building down to ground floor level, keeping one of the external walls as it is a party wall. The existing deep foundation piers are to remain. As the building has an extensive foundation structure, i would estimate over half of the buildings original mass is to remain. The upgrade involves a new 2 storey lightweight timber/steel super structure, to be supported off the existing ground floor and foundation structure. 

My question is: What capacity parameters should be used for the existing structure? Are probable capacities okay to use (like in assessment) or should design capacities be used (as for new design). As the result is quite significant, especially for the soil interaction which generally goes from a design reduction factor of 0.5 to a probable which is set at 1.0 (as per section C4.5.2 of MBIE guidelines 2017).

The new design component above ground floor, will be done with design capacities, that is clear to me. 

Interest to hear if anyone else has asked themselves this question before and where the line sits between strengthening and new design. Appreciate any feedback.