Does a Change of Use Trigger Title 24 in California?
Yes. Where an alteration changes the occupancy classification of a space, the Energy Code requirements apply to the new occupancy type — so a space that complied under its old use does not automatically comply under the new one.
The requirements follow the new use
This is the whole point. For alterations that change the occupancy classification of a tenant space, the Section 141.0 requirements apply to the new occupancy type. A shell that was compliant as a shop is assessed as a restaurant the moment it becomes one, and nothing about its previous compliance carries over.
Why it usually costs more than a like-for-like fit-out
Because a change of use tends to move several systems at once. Lighting power allowances are set per primary function area, so reclassifying the space changes the permitted total. Ventilation rates follow the use. A kitchen brings exhaust and make-up air that a retail shell has none of. Each of those is a regulated component, and touching any of them makes the work an alteration under Section 141.0(b).
The case that is not an alteration at all
Worth separating out, because it changes the answer. A first-time tenant improvement in a space that has never been occupied or used meets the definition of a newly constructed building rather than an alteration. So fitting out a brand-new shell for its first tenant is new construction, while changing the use of a space that has been occupied before is an alteration.
Establishing which of those describes your project is the first question, not a detail.
Conditioning previously unconditioned space
A third route again. Bringing space that was never heated or cooled into conditioned use is newly conditioned floor area, which is treated as an addition rather than as an alteration. A change of use that also conditions a former warehouse bay is doing two things at once.
What to establish before design
The old and new occupancy classifications, which systems the change touches, and whether any previously unconditioned area is coming into use. Those three answers determine the compliance route, the forms and whether acceptance testing applies — and all three are cheaper to settle on a drawing than after a plan check comment.