Does remodeling part of my house make the whole house meet Title 24?

No: under California's Energy Code, an alteration only has to bring the components you are changing up to code; the rest of the house can stay as it is. The California Energy Commission's 2025 compliance manual confirms that existing lighting does not have to be replaced when other lighting is altered, and that unaltered components do not have to be brought into compliance. Repairs are outside the Energy Code entirely.

What counts as an alteration

The manual defines an alteration as any change to a water heating system, space conditioning system, lighting system or envelope that is not an addition. Its examples include replacing windows, adding insulation, reroofing, replacing a furnace or air conditioner, extending or replacing ducts, replacing a water heater, and replacing or adding hardwired light fixtures.

How compliance works for alterations

• Prescriptive. Each altered component meets the alteration requirements in Section 150.2(b), documented on a prescriptive certificate of compliance. Every applicable prescriptive requirement must be met.
• Performance. If one alteration falls short, the existing-plus-alterations approach can trade it off against other alterations that exceed requirements. Credit for existing conditions may require third-party verification.
• As new construction. Modeling the whole house as if it were new is allowed but is the hardest route, since every feature must meet current code.

Examples from the manual

• A remodel that only installs new faucets and bathroom lighting must meet just the applicable mandatory appliance and lighting requirements.
• Replacing a recessed light in a kitchen means the new fixture must be high efficacy and cannot use a screw-base socket. Existing recessed cans with screw-base sockets can stay if you install compliant trim kits or lamps.
• An entirely new lighting system for a whole-home renovation is treated like lighting in a newly constructed building.
• Replacing an electric resistance water heater with a new electric water heater that meets the appliance efficiency rules is allowed; new piping must meet the pipe insulation requirements.
• Replacement windows must meet the prescriptive U-factor limits, which are 0.27 in climate zones 1 through 5, 11 through 14 and 16, and 0.30 in the others.

Where small changes trigger bigger work

Some alterations reach further than you might expect. Replacing an air conditioner's outdoor unit triggers duct sealing and testing of the existing ducts, and altering a ceiling can trigger attic insulation requirements in many climate zones unless an exception applies, such as knob-and-tube wiring in the attic.

What this means for your MEP plans

Your plans should make clear which systems and fixtures are new, replaced or existing to remain, so the reviewer can apply the right rules to each. MEP Plans USA prepares residential MEP plans and Title 24 reports for California remodels. See residential MEP plans, Title 24 reports and how Title 24 affects lighting. Local amendments, including reach codes, can add requirements, so confirm with your building department.

Sources: energy.ca.gov.