Does an ADU Need Solar Under Title 24?
A newly constructed ADU generally does. An ADU created by converting space that already exists generally does not, because the code treats that as an addition rather than a new building.
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Where the requirement comes from
The Energy Code requires a solar photovoltaic system on newly constructed single-family residential buildings, and that category includes accessory dwelling units. So a detached ADU built new on a lot is in scope from the start, and the array is sized against the building rather than being a token installation.
The requirement carries a short list of exceptions. They exist for situations where a compliant array is not physically or practically achievable — limited or shaded roof area being the common one. An exception has to be established for the specific project; it is not a general opt-out.
Conversions are outside it
An ADU made from an existing garage, shed or other unconditioned structure is newly conditioned space, which the code treats as an addition. The solar requirement does not attach to it. This is the single largest practical difference between building a new detached unit and converting something already standing, and it is worth establishing before the design is committed rather than after.
Using the array that is already there
Where a new ADU does trigger the requirement and the main house already has solar, the requirement can be satisfied by adding modules to that existing system rather than installing a separate one. Two conditions apply: the added modules have to be on the same residential lot as the ADU, and the load-serving utility has to permit the expansion.
That route is usually cheaper and simpler than a standalone array, but it depends on the utility's position and on the existing system having headroom, so it is confirmed rather than assumed at design stage.