And yes, For buildings under the [HCD 1 & HCD 2]; CA requires that the ABS be limited to two floors
I have done several buildings with "mixed" piping in several jurisdictions.
I'm sure the LA building department would have known if there were a section that makes that limitation.They did not tag it in plancheck or in the field.
The underlying intent??? 1. A concern with plastic piping that is more than two stories in height that fails over time due to induced stresses... how aggregate vertical loads affect the pipe and connections, resulting in cracking/leaks... expansion/contraction/movement in long pipe runs, how this is handled by supports and connections/collars, and other technical issues. 2. Pressure from plumbers unions and pipe manufacturers to write the code in a way that seemingly removes discretion, forcing installation of 'all iron' systems even when combination systems might perform as well.
If a BO interprets the code to mean 'if more than 2 stories all portions must be iron', then you always have the option of presenting an 'alternative' mixed-materials design. You could explain that no portion of the plastic piping will carry loads more than two stories in height. You could convince the BO that there is no reason the plastic portions should be disallowed. You could search and find printed materials that support your interpretation.
You would probably have a hard time explaining to an owner/client why it's going to take longer to get his plans approved - even when you explain that there will be some dollar savings. You would probably also need to get some preliminary estimates from contractors to show your client the magnitude of savings obtained.
The 'alternative design' process could be costly and time consuming. If you have a cooperative BO it might be a breeze. On the other hand, you could have a BO that will insist on documentation - as is the BO's discretion under the code. The documentation could be simple or complicated. It's a crap shoot.
The best time to talk to a BO about such issues is when the design is still in preliminary stages. Most BO's will appreciate a designer coming in early to vet anything that could be a concern (i.e. out of their ordinary, daily experience). It's simply a part of this complicated business we all have to deal with - no matter how simple we 'think' it should be.