Title 24 addition requirements in California decide how much insulation, how much glass and what kind of heating and testing a new room on a mid-century house needs. The rules are in the 2025 Building Energy Efficiency Standards (the Energy Code, Title 24, Part 6). The California Energy Commission says buildings whose permit applications are applied for on or after January 1, 2026 must comply with the 2025 code.
This guide covers what counts as an addition, the size-based rules in section 150.2(a), the performance approach, what used to be called HERS verification, and the forms. It draws on the code, the Energy Commission's 2025 residential compliance manual and a city page read in November 2026. Rules change and cities can add local amendments, so confirm with your own building department.
Addition, alteration or repair?
The Energy Code sorts work three ways (section 100.1), as of November 2026:
Addition: any change that increases both conditioned floor area and conditioned volume. The compliance manual's examples include converting a garage into living space, enclosing and conditioning a patio, and legalizing conditioned space that was added without a permit.
Alteration: any change to the water heating, space conditioning, lighting, electrical power distribution system or envelope that isn't an addition. The manual treats a loft built inside an existing vaulted ceiling as an alteration, because it adds floor area but not volume.
Repair: reconstruction or renewal for maintenance. Replacing a component the code regulates is an alteration, not a repair.
The manual says the Energy Code applies to additions and alterations but not repairs. Reroofs, replacement windows and heating swaps are alterations with their own rules; see our guides to Title 24 reroof requirements and heat pumps for mid-century homes.
What every addition must meet
Section 150.2(a) says an addition must meet the mandatory measures in sections 110.0 to 110.9 and most of section 150.0, plus either the prescriptive approach (150.2(a)1) or the performance approach (150.2(a)2). Its exceptions matter for small projects:
Additions of 300 square feet or less don't have to meet the prescriptive roofing (cool roof) requirement.
When heating or cooling is extended from the existing system, the existing equipment doesn't have to be upgraded, but its heating capacity must be adequate under the Residential Code.
When any length of duct is extended from an existing duct system, the existing and extended ducts must meet the duct insulation and sealing rules for altered ducts.
Additions of 1,000 square feet or less don't need the ventilation cooling (whole-house fan) requirement.
Solar panels (photovoltaic systems) aren't required for additions.
Prescriptive rules by addition size
As of November 2026, the prescriptive approach in section 150.2(a)1 sets two size bands.
Over 700 square feet
The addition meets the prescriptive package for new houses in section 150.1(c), with changes.
Extensions of existing wood-framed walls may keep the existing wall dimensions with R-15 cavity insulation in 2x4 framing or R-21 in 2x6 framing.
Total glass (fenestration) is capped at the greater of 175 square feet or 20 percent of the addition's floor area, and west-facing glass at the greater of 70 square feet or the section 150.1(c) limit.
700 square feet or less
Attic insulation over a ventilated attic: in Climate Zones 3 and 5 through 7, an assembly U-factor of 0.031 or less (R-30 in wood framing); in Climate Zones 1, 2, 4 and 8 through 16, 0.025 or less (R-38).
Radiant barriers in Climate Zones 2 through 15.
The same R-15 or R-21 rule for extended wood-framed walls.
Glass: the code's glass item opens with "In Climate Zones 2, 4 and 6-15," then caps west-facing glass at 60 square feet and adds a total-glass limit: the greater of 120 square feet or 25 percent of the addition's conditioned floor area for additions over 400 square feet, or the greater of 75 square feet or 30 percent for 400 square feet or less. Ask your energy consultant how these limits apply in your zone.
Quality Insulation Installation does not apply.
The compliance manual notes that a 400-square-foot addition gets the 30 percent glass limit when it complies as existing plus addition, but only 20 percent when it complies as the addition alone. Check your climate zone first; our reroof guide explains how.
Ventilation, water heaters and system sizing
Ventilation: additions that grow the house by 1,000 square feet or less don't need the whole-dwelling ventilation airflow; larger ones size it on the whole house. Additions still follow the local exhaust rules, which require exhaust in each kitchen and bathroom (section 150.0(o)1G).
Water heater: if an additional water heater is installed with the addition, section 150.2(a)1D lists the types allowed, including heat pump water heaters that meet set conditions and, for additions of 500 square feet or less, an electric water heater with point-of-use distribution.
Sizing: heating and cooling capacity limits apply unless airflow is field verified at 350 cfm per ton or more; ductless systems are excepted.
The performance approach
Under section 150.2(a)2, a designer models the project in Energy Commission-approved software, with the energy budget expressed in the code's long-term system cost (LSC) metric. The compliance manual describes three ways to run it:
Addition alone: only the addition is modeled. Little information about the house is needed, but some prescriptive allowances don't apply and improvements to the existing house can't be traded off.
Existing plus addition plus alteration: the whole house is modeled, and unaltered parts don't have to be brought up to code. The manual says it lets improvements to the existing house count toward compliance. Under the code, existing framed walls with at least R-11 insulation don't have to meet the mandatory wall insulation rule (section 150.0(c)) in this approach.
Existing plus addition as a newly constructed building: used when changes are extensive. The manual says it can be difficult because all existing features must meet current code.
What HERS verification is now
Some energy features have to be field verified and tested by a third party. For years that was the Home Energy Rating System (HERS) program. The Energy Commission's HERS page now says that, effective January 1, 2026, the HERS program no longer supports Energy Code compliance, and the field verification and diagnostic testing provisions now appear in the Energy Code.
The 2025 code calls this the Energy Code Compliance (ECC) program. An ECC-Provider is an organization approved by the Commission to run it, and an ECC-Rater is a person trained, tested and certified by a provider to do the field verification and testing. Under section 10-103.3, raters must not have a financial interest in the builder, designer or subcontractor who signs the compliance documents, unless the rater company files a declaration of separation of services under that section.
Ducts are one place an addition can need a rater. When more than 25 feet of new or replacement ducts are installed, or ducts are extended to serve an addition, the altered system must be sealed and tested. For an extension, section 150.2(b)1D sets three options: total leakage of 10 percent or less of air handler airflow, leakage to outside of 7 percent or less, or, if neither is possible, sealing all accessible leaks, confirmed by an ECC-Rater's visual inspection and smoke test. Existing duct systems constructed, insulated or sealed with asbestos are excepted from that sealing rule when they are extended; our asbestos survey guide covers testing before a remodel. The compliance manual adds that the 2025 prescriptive package for new houses (section 150.1(c)7A) requires refrigerant charge verification for heat pumps in all climate zones.
The compliance forms
The Energy Commission's 2025 residential compliance forms page lists three certificate types: the Certificate of Compliance (CF1R), the Certificate of Installation (CF2R) and the Certificate of Verification (CF3R). For additions, as of November 2026, it lists:
CF1R-ADD-01-E: Prescriptive Residential Additions 1,000 Square Feet or Less (a registered form).
CF1R-ADD-02-E: Prescriptive Residential Additions That Do Not Require ECC Field Verification.
CF1R-NCB-01-E: Prescriptive Newly Constructed Building and Additions Greater than 1,000 Square Feet.
The compliance manual says the performance approach produces a CF1R-PRF-01-E. Forms in the registered category must be registered with an ECC-Provider before they go to the building department. Under section 10-103(a), the person eligible under the Business and Professions Code to take responsibility for the design signs the CF1R; the person in charge of the installation signs the CF2R; and the ECC-Rater who did the testing signs the CF3R. For additions under 300 square feet, and alterations such as window or water heater replacements, that need no field verification, the city may skip the CF1R and CF2R or use simplified forms. Under section 10-103(b), the builder leaves copies of the CF1R, CF2R and CF3R in the house for the owner at occupancy, so buyers can ask a seller for them.
City rules on top of the state code
Cities can add local energy rules. Palo Alto's page for its 2026–2028 green building and local energy code update is still titled "Proposed," but its timeline lists the City Council's second reading and adoption on October 27, 2025, with adopted reach codes going into effect January 1, 2026. The page says the city's green building ordinance applies to additions and alterations. For houses it lists a heat pump or extra efficiency measures when an air conditioner is replaced, a flexible set of efficiency and electrification measures for certain remodels, and, starting in 2027, a heat pump water heater (or a solar thermal alternative) when a gas water heater is replaced. Ask Palo Alto, or your own city, which local rules apply before you design.
This is general information as of November 2026, not legal advice. Check with your city's building department and a licensed contractor or designer before you plan an addition.
Frequently asked questions
Is a garage conversion an addition under Title 24?
Yes. The compliance manual lists converting a garage or other unheated space into living space as an addition, because it adds conditioned floor area and volume.
Do I need solar panels on an addition?
No. Section 150.2(a) says photovoltaic systems aren't required for additions.
Do I still need a HERS rater?
You may need an ECC-Rater, the code's new name for the field verification role. If the addition extends ducts or adds a heat pump, expect testing. A small addition with no field-verified features may need only simplified forms.
Related reading
Mid-Century Modern Window Replacement: Retrofit, Film and Title 24
Hire a Licensed Contractor in California: Mid-Century Remodel Guide
More guides on this topic: Owning & Renovating a Mid-Century Modern Home
Planning an addition to a mid-century home you own or want to buy? Call or text the Boyenga Team at Compass at (408) 373-1660 or (650) 383-8606, or contact the Boyenga Team to talk it through.
