Using the CA res code, see 313.2 and definition of "alteration". For replacing a structure on top of an existing fire damage foundation (hopefully certified by an engineer as usable) it appears the code language is direct using "alteration". The fire damage reconstruction requires a permit thus must comply with current code so fire sprinklers are required. Verify you do not have local ordinance that modifies the code section or conflicts with the CA IRC.
If your jurisdiction has adopted appendix J for existing buildings, there are general requirements for "renovation" that will require compliance to code in effect for 50% or more repaired.
The res code section 184.108.40.206 provides that ch 34 of the "building code" prevails for existing buildings. Sections 3403 3404 & 3405 cover additions alterations and repairs. Generally new work must comply with current code and if the structure is damaged sufficiently to require replacement, then the current code provisions prevail.
A partial reconstruction is not directly addressed for percentage trigger in ch 34 (at least that I can find). As a matter of policy, it is a good idea to add provisions in the adopting ordinance that address issue such as fire damages or damages/reconstruction with the appropriate triggers stated and clearly defined for level such as percentage of area, percentage of building cost, etc.