How does the 2025 Energy Code apply to a home addition in California?

An addition is treated much like new construction for the new space, with some allowances based on its size, and you can show compliance for the addition alone or for the whole house. The California Energy Commission's 2025 compliance manual defines an addition as any change that increases both conditioned floor area and conditioned volume, including conditioning a space that was not conditioned before. The certificate of compliance (CF1R) is submitted with the permit application.

What counts as an addition

The manual's examples include a new room or sunroom, converting a garage or other unheated space into living space, enclosing and conditioning a patio, and legalizing a previously unpermitted conditioned addition. Adding a loft inside the existing volume is an alteration, not an addition.

Three prescriptive size tiers

• 400 square feet or less. Glazing up to 75 square feet or 30 percent of the added floor area, whichever is greater; quality insulation installation (QII) does not apply.
• Over 400 up to 700 square feet. Glazing up to 120 square feet or 25 percent, with a west-facing limit in many climate zones; QII does not apply.
• Over 700 square feet. Glazing up to 175 square feet or 20 percent, and QII applies to the addition.

Other allowances: additions of 300 square feet or less skip the cool roof requirement, and the whole-house fan and indoor air quality fan requirements do not apply to additions of 1,000 square feet or less, with different ventilation rules when an addition creates a new dwelling unit. Solar PV requirements do not apply to additions or alterations.

Choosing a compliance approach

• Addition alone. Only the new space is modeled. Little information about the existing house is needed, but some allowances are tighter and you cannot trade off improvements to the existing house.
• Existing + addition + alteration. The whole house is modeled and unaltered components do not have to be brought up to code. It is the most flexible option but needs data on the existing house.
• Existing + addition as new construction. The hardest route, since every existing feature must meet current code.

MEP items that change for 2025

• Load calculations for additions must use an envelope leakage value no higher than Table 150.2-C unless leakage is tested, and block loads may be used for sizing.
• Gas tankless water heaters were removed from the prescriptive options when an addition adds a water heater; a heat pump water heater is the usual prescriptive choice, with an electric option for additions of 500 square feet or less.
• Local exhaust for bathrooms and kitchens is required in any addition.
• If any feature needs ECC field verification (the program formerly called HERS), the CF1R must be registered with an ECC provider before submittal.

What this means for your MEP plans

Decide on the compliance approach early, because it determines how much existing-house information your designer needs and which HVAC and water heating options work. MEP Plans USA prepares residential MEP plans and Title 24 reports for California additions, delivered as PDFs with a standard turnaround of 5 business days. See residential MEP plans, Title 24 reports and prescriptive vs. performance compliance. Local amendments vary, so confirm with your building department.

Sources: energy.ca.gov, energy.ca.gov.