Only question is: how do I prove that?
It would be 4 separate office suites per floor, for different tenants. Because of that, does the second floor need to be accessible since those 4 spaces wouldn't be accessible?
Now you are asking a different question than your original question. The the answer to the original question is the 20% limitation, as mentioned above by arwat23. I am no longer licensed as an architect in California, so I don't know how they calculate the 20% in California. In general, though, what you need to understand is that the 20% rule is not a "get out of jail free" card. It doesn't mean that if an elevator would cost more than 20% of the total construction cost you don't have to do anything. As walker.t points out, "accessibility" means a lot more than an elevator.
The 20% rule in the IEBC is a codification of the ADA provision that says the cost of creating accessibility in exiting buildings shall not be unreasonable (or maybe the ADA word was "disproportionate" -- don't remember.) The ADA basically said that, but didn't in any way quantify where to draw the line between "reasonable" and "unreasonable." Over the years, courts settled on 20% as being a reasonable limit on accessibility improvements, and the ICC codes long ago put that into the code. But it doesn't say that if an elevator will cost more than 20% you don't have to do anything.
The 20% rule starts with the requirement that any space you alter or change the occupancy of must be made accessible. The 20% rule then addresses the accessible route TO the affected space(s). The accessible route begins at the accessible parking, continues across the site through the accessible entrance, and then follows through the building to the affected space(s). By definition (unless California changed it), the accessible route includes the toilet rooms and drinking fountains serving the affected space(s). So if an elevator will cost more than 20%, unless everything about the accessible route to the affected spaces is already 100% compliant, you are required to spend "up to" 20% of the construction cost on improving other aspects of the accessible route.
The theory is that, while it may be unreasonable to require a small renovation to make everything about a building 100% accessible in one go, if several small projects over the years each make some improvements, eventually the building may be made 100% accessible -- which is the goal of the ADA.
How do you prove that you are spending 20% on accessibility upgrades? You submit a detailed cost breakdown for the project, showing ALL construction costs and highlighting those costs that apply toward accessibility upgrades.