What it is
Title 24 is California's set of energy efficiency standards for buildings. It governs how much energy a building is permitted to use, and it applies to new construction and, above certain scopes, to remodels.
For homeowners it usually surfaces as a constraint on three things: glazing, insulation, and mechanical equipment.
Where it bites in a remodel
Glazing is the most visible. Large windows and sliding walls are exactly what people want in Southern California, and they are also where energy performance is hardest to achieve. The answer is rarely fewer windows - it is better specification, shading, and orientation-aware design.
Insulation comes next. Once walls or ceilings are open above a certain scope, bringing insulation up to standard stops being optional. This is generally a good thing in a Valley house, where summer cooling load is significant.
Mechanical equipment is the third. Replacing an HVAC system brings efficiency requirements, and duct sealing and testing often come with it.
The compliance path matters
There is more than one route to demonstrating compliance, and which one you use affects design freedom. A performance-based approach models the building as a whole and allows trade-offs - more glass here, better insulation there - where a prescriptive approach applies fixed requirements element by element.
That flexibility is the argument for bringing energy compliance into schematic design rather than treating it as paperwork at the end. Designs that ignore it until plan check are the ones that get redrawn.
The upside
It is worth saying plainly that these requirements mostly produce better buildings. A Valley house with proper insulation, decent glazing and a right-sized system is cheaper to run and considerably more comfortable in August. The friction is nearly always about sequencing, not about the standard itself.
Bring Title 24 in at schematic design and it shapes the building sensibly. Leave it to plan check and it redraws your windows for you.