Can a California project comply using the IECC?

No. The Building Standards Commission says model codes cannot be used in place of Title 24, and Part 6 is not based on a model code. Energy compliance is shown on Part 6 forms.

Is Title 24 stricter than the IECC?

There is no single official ranking to quote. DOE compares state codes with the model codes through an energy index, and for a code that differs fundamentally its method allows it to rely on analysis by others.

Who publishes the printed California Energy Code?

The International Code Council, according to the Building Standards Commission. The Energy Commission also publishes the 2025 Standards as a free PDF.

Title 24 vs the IECC: What Is the Difference?

The IECC is the International Code Council's model energy code, the one the federal government measures residential codes against. California does not adopt it: Title 24 Part 6 is a California-only code written by the Energy Commission under the Warren-Alquist Act, and the energy provisions of the model residential code are not adopted here.

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Two codes with different authors

The International Energy Conservation Code is written by the International Code Council, which has developed an energy code for more than three decades. The U.S. Department of Energy treats it as the national model energy code for residential buildings, with ASHRAE Standard 90.1 as the model for commercial ones.

Most of California's building code is built on model codes. The Building Standards Commission lists the California Building Code as based on the International Building Code, the Residential Code on the International Residential Code and the Fire Code on the International Fire Code. Part 6 is different. The Building Standards Commission names Parts 1, 6, 8, 11 and 12 as California-only codes that are not based on model codes. Part 6 is the Energy Commission's own text.

The missing chapter in the Residential Code

This is where designers from other states get caught. Part 2.5, the California Residential Code, is based on the International Residential Code, but the Building Standards Commission says the energy conservation provisions of that model code are not printed in Part 2.5 and are not adopted for use in California. A house designed to the Residential Code therefore takes its energy rules from Part 6 and nowhere else, and an IECC compliance report is not a California compliance document. The plan checker expects a certificate of compliance produced in Energy Commission-approved software.

There is an irony in the publishing. The Building Standards Commission lists the International Code Council as the publisher of the printed Part 6. The ICC prints California's Energy Code; it does not write it.

Why California keeps its own

The Warren-Alquist Act of 1974 gave the Energy Commission the job of adopting and maintaining building energy standards, and it sets the tests they must pass. The standards must be cost-effective "when taken in their entirety and amortized over the economic life of the structure", in the words of the Nonresidential Compliance Manual. The Public Resources Code also requires performance standards expressed as an energy budget per square foot. The Commission sets that budget for each building type in each of California's 16 climate zones, rather than adopting a national text and amending it.

How Washington lines the two up

Federal law still asks California to look at each new IECC. When the ICC publishes an edition, DOE has 12 months to decide whether it improves residential efficiency. If it does, each state must certify within two years that it has compared its own code with it. DOE issued its determination on the 2024 IECC on 20 December 2024, estimating national site energy savings of about 7.8 per cent, and set 30 December 2026 for state certifications.

DOE's California profile lists the 2025 Building Energy Efficiency Standards as the state code for both residential and commercial buildings, adopted on 17 December 2024 and effective on 1 January 2026. Its method scores residential codes on an energy index relative to the 2006 IECC, and for codes that differ fundamentally from the model codes, its national laboratory may rely on analysis by others.

Where the 2006 IECC still appears

The 2006 edition lives on as a yardstick. DOE's residential index is built on it, and RESNET sets 100 on its HERS Index at a typical home built to 2006 standards; see Title 24 vs the HERS Index. For what Part 6 itself asks of a project, start with Title 24 requirements.