Building Types | Energy Code

California's 2025 Energy Code for a New Duplex or Fourplex

California's Energy Code sets how much energy a new building may use and what equipment the code assumes. A new version, the 2025 Building Energy Efficiency Standards, applies to every permit application filed on or after January 1, 2026. This page explains the parts that apply to a small multiplex: how the code sorts a duplex, triplex or fourplex, when solar panels are required, what the heat pump baselines mean, and how the rules reach accessory dwelling units (ADUs) built under state law such as SB 9 (2021).

What the 2025 Code Is and When It Applies

The Energy Code is Title 24, Part 6. The California Energy Commission (CEC) adopted the 2025 update at its September 11, 2024 business meeting, and its adoption release said the update would go to the California Building Standards Commission, "which is scheduled to consider it in December 2024." The official document is CEC-400-2025-010-F, dated July 2025.

The start date depends on your permit filing. The CEC's 2025 standards page says: "Buildings whose permit applications are applied for on or after January 1, 2026, must comply with the 2025 Energy Code." Its fact sheet adds: "Local building departments start enforcing the 2025 Energy Code on January 1, 2026." A project that filed for its permit before that date follows the earlier code.

The CEC says the 2025 code expands heat pumps for space and water heating in new single-family, multifamily and some nonresidential buildings, adds rules that prepare buildings for electric equipment (the CEC calls them electric-ready), and strengthens ventilation in multifamily buildings. It estimates the update will provide "over $4.8 billion in statewide energy cost savings over 30 years." That figure is the Commission's own estimate.

Is Your Building Single-Family or Multifamily?

The code sorts buildings first, and the rules follow from that. Section 100.1 of the 2025 Standards says: "SINGLE-FAMILY BUILDING is any of the following: A residential building of Occupancy Group R-3 with two or less dwelling units, ... A townhouse." It also says: "MULTIFAMILY BUILDING is any of the following: A building of Occupancy Group R-2, other than a hotel/motel building or timeshare property."

An occupancy group is the building code's label for how a building is used. The Energy Code counts an R-3 building with two or fewer homes as single-family, so a new duplex follows the single-family rules. Whether a triplex or fourplex is R-2 or R-3 is decided under the building code (Title 24, Part 2). If it is R-2, it is a multifamily building under the Energy Code. Confirm the occupancy group with your building department early in design.

Sources: 2025 Standards, Sections 100.1, 150.1(c)14 and 170.2(f), CEC fact sheet and the HCD ADU Handbook, page 38.

Solar on a New Duplex or Townhouse

Solar panels are called a photovoltaic or PV system in the code, and their size is measured in kilowatts of direct current (kWdc). Section 150.1(c)14 says: "All single-family residential buildings shall have a newly installed photovoltaic (PV) system." The required size is the smaller of two numbers: the result of Equation 150.1-C in the code, or the solar access roof area (SARA) multiplied by 18 for a steep roof, or by 14 for a low-slope roof. SARA is the roof area that qualifies for panels.

Two exceptions remove the requirement. "No PV system is required when the minimum PV system size specified by section 150.1(c)14 is less than 1.8 kWdc." And "No PV system is required if the SARA is less than 80 contiguous square feet." On steep roofs, north-facing areas (azimuth between 300 and 90 degrees, the compass direction the roof faces) do not count toward SARA. A small duplex with a north-facing steep roof can therefore fall under the threshold. Your energy consultant runs the equation for your design.

Solar on a New Triplex or Fourplex

Multifamily buildings follow Section 170.2(f): "All multifamily buildings up to three habitable stories shall have a newly installed photovoltaic (PV) system." The exception: "No PV system is required when the minimum PV system size specified by Section 170.2(f) is less than 4 kWdc." The 4 kWdc threshold for multifamily buildings is higher than the 1.8 kWdc threshold for single-family buildings.

The section heading covers buildings of three habitable stories or fewer. SB 9 homes, for example, can be held to local height rules such as San Jose's 30 feet or two stories, which keeps them inside that range. For a taller building, ask your energy consultant which section applies. The CEC also issued a corrections notice on June 9, 2025 about battery storage capacity requirements; ask your energy consultant whether battery rules apply to your building.

How the Layout Changes the Energy Rules

The same number of homes can fall under different Energy Code rules depending on how you arrange them. Here is how the definitions above apply to the layouts the state housing laws allow on one lot.

• Two detached houses under SB 9. Each house is its own single-family building, so each one gets the single-family solar rule and its own 1.8 kWdc and 80 square foot exceptions.
• One duplex building classed R-3. One single-family building with two homes, under the same single-family solar rule.
• A lot split with a duplex on each new lot. Two buildings, each a single-family building if classed R-3 with two homes.
• One fourplex building classed R-2. One multifamily building under Section 170.2(f), with the 4 kWdc exception, and the heat pump water heater baseline where each unit has its own water heater.
• A house plus ADUs. Each newly built detached ADU carries the solar requirement, with panels on the ADU or the house. A converted garage or other existing space does not.

Ask your designer to check the Energy Code category for each layout you are comparing. The number of buildings, and whether each is R-3 or R-2, decides which solar section and which water heating baseline apply.

Heat Pump Baselines

The CEC fact sheet describes the change in heating: "For homes, use heat pumps for both space heating and water heating, expanding on the single heat pump baselines in the 2022 update." For low-rise multifamily buildings with a water heater in each unit, the baseline is a heat pump water heater. A heat pump is electric equipment that moves heat from one place to another.

A baseline is the equipment in the standard design the code uses for comparison. The prescriptive rules, the code's list of set measures, name heat pumps. One listed option for single-family water heating in Section 150.1(c)8.A is "A single 240 volt heat pump water heater (HPWH)." The CEC adds a reminder: "Reminder: The CEC does not mandate specific fuel types." Your designer or energy consultant shows how your plans comply.

The prescriptive heat pump requirement is set out by climate zone, in tables such as Table 150.1-A. Ask your energy consultant which entries apply to your lot.

ADUs and the Energy Code

HCD's ADU Handbook (page 38) explains how the solar rule reaches ADUs: "newly constructed ADUs are subject to the California Energy Code requirement ... to provide solar systems if the unit(s) is a newly constructed, detached ADU (though some exceptions apply)." The panels can go on the ADU or on the main house. ADUs converted from existing space, and ADUs built as additions, are exempt from the solar rule.

This matters for the unit counts in this hub. A lot that adds a new detached ADU under state ADU law, and from January 1, 2027 two detached ADUs under AB 956, adds a solar requirement for each new detached unit. A garage conversion does not. For Energy Code questions on ADUs, HCD lists the Energy Standards Hotline at 800-772-3300 and [email protected].

Local Rules Can Go Further

Cities and counties may adopt ordinances that exceed the Energy Code. The CEC's 2025 standards page keeps a list titled "Local Ordinances Exceeding the 2025 Energy Code." Before you design, check that list for your city and ask the building department whether it has adopted extra electric or solar rules. The rest of your project's rules come from zoning: see the duplex to fourplex page for how many homes a lot can hold.

Where to Go Next

Related on VanPlex: the SB 9 guide and the San Jose page, where SB 9 homes are capped at two stories.

Frequently Asked Questions

Official sources (checked October 2026)