Yes. California building departments allow an ADU's Title 24 PV requirement to be satisfied by a system installed on the main house, the ADU, or split across both — as long as the combined system meets the total required kilowatts. This is the most common compliance path when the ADU has limited roof area.
Your Title 24 documentation (CF1R form) calculates the required PV size based on conditioned floor area for each building. If the ADU requires 1.5 kW and the main house has space for an extra 1.5 kW of panels, the entire system can live on the main house roof.
If the ADU shares the main house electric meter, the solar offsets both buildings' usage automatically. If the ADU has a separate meter (common for rentals), the inverter can be wired to feed either meter — your installer chooses based on rate structure and tenant arrangement.
If the existing system is large enough to cover the new combined requirement, no additional panels may be needed. Otherwise, you'll add panels to the existing system.
Adding panels to an existing pre-NEM 3.0 system can sometimes preserve your legacy NEM 2.0 status if the addition stays under specific size thresholds. This is highly time-sensitive — ask us before adding capacity.