All newly constructed attached ADUs can be complied as Addition. Thus they don't need solar.
All conversions from unconditioned space (such as garage or pool house or sheds) to ADUs, be it attached or detached, can be complied as Additions. Thus they don't need solar. In fact, even if a ADU is being built new and atop or attached to a garage in any way, it still can be complied as an Addition.
All conversions from conditioned space to ADUs, can be complied as an Alteration. Thus they don't need solar.
All newly constructed detached ADUs need to be complied as new constructions. Thus they need solar.
The Title 24 compliance requirements in terms of stringency are as new constructions > additions > alterations/existing. ie. new constructions have the most stringent Title 24 requirements (and hence tend to be most costly), followed by additions (medium cost) and then alterations/existing (lowest cost). Hence it's always preferable to comply as alteration or addition over new construction.
All ADUs need to mandatorily meet the IAQ ventilation requirements of the Title 24 energy code. Hence they also mandatorily require IAQ HERS testing on completion.
Below are attached the key bulletins referenced above.