Title 24 for a Commercial Building Addition
An addition can comply two ways: prescriptively, where everything new meets the applicable requirements on its own, or on the performance path, where either the addition alone or the whole existing building plus the addition is modelled against the energy budget.
An addition is new area, not altered area
The distinction drives everything. An alteration changes something that exists; an addition creates conditioned floor area that was not there. That is why an addition is assessed close to how new construction is assessed, while an alteration is assessed only on what changed.
One case regularly gets classified wrong: bringing space that was never heated or cooled into conditioned use. That is newly conditioned floor area and is treated as an addition, even though nothing was built. Conditioning a former warehouse bay is an addition in code terms.
Route one: prescriptive
Everything new in the addition meets the applicable requirements on its own — envelope, lighting, mechanical systems, outdoor lighting and any new signs, against the mandatory and prescriptive sections that would apply to that work.
This is the simpler route to document and the harder one to satisfy on a constrained design, because there is nothing to trade. Every component has to clear its own bar.
Route two: performance, in two forms
The performance path models the design against an energy budget, and there are two ways to draw the boundary. Either the addition alone complies, or existing plus addition plus alteration complies as a whole.
The second form is the one worth understanding, because it changes what a project can do. Upgrades to the existing building can be counted toward the addition's compliance. An addition with a heavily glazed elevation that cannot pass on its own may pass comfortably if the existing building's lighting is being upgraded in the same permit.
Choosing between them
Prescriptive suits a straightforward addition with conventional glazing and no unusual systems — it is quicker and cheaper to document. Performance earns its cost where the addition has an architectural feature that will not pass on its own, or where existing-building work is already in scope and can be credited.
The existing-plus-addition form specifically requires deciding the boundary before modelling starts, since it determines what gets surveyed and documented in the existing building. That is a scoping decision, not a modelling one.
Establish this first
Whether every part of the scope is genuinely an addition, whether any of it is alteration or newly conditioned existing area, and whether existing-building upgrades are in the same permit. Those three answers pick the route — and picking the route late means either redocumenting or remodelling.