ADUs · Additions
Does my addition or ADU have to be all-electric now?
No, California did not ban gas. It did change what the code measures you against, and one line about water heaters catches most additions by surprise.
Not quite, but the default moved. The 2025 Energy Code (Title 24, Part 6) applies to every permit application filed on or after 1 January 2026. For a new detached ADU it treats the unit as a newly constructed home, and the prescriptive package for a new home now has a heat pump for space heating and a heat pump water heater as the baseline. A gas furnace is still legal, but only through the performance path, where the energy budget is set by the heat pump design. For an addition the existing furnace can usually be extended, and solar is not required. The line that surprises people: if the addition gets its own new water heater, it has to be a heat pump water heater.
Which rulebook applies to your project
The code that governs your plans is the one in effect on the day the building permit application is submitted, not the day you sign a contract or the day work starts (Health and Safety Code §18938.5, as the Building Standards Commission explains in its own FAQ). The 2025 edition of Title 24, including the Energy Code, took effect on 1 January 2026. A plan set drawn in 2025 against the 2022 code and filed in 2026 gets checked against 2025.
The second thing to settle is how the Energy Code classifies the work, because three different sets of rules hang off that one word. The California Energy Commission's own ADU guidance (2025 Energy Code ADU FAQs) draws the lines.
| What you are building | Energy Code category | What that means |
|---|---|---|
| A new detached ADU in the yard | Newly constructed building (§100.1) | The full new-home package: prescriptive heat pump baseline, heat pump water heater, solar PV sizing, electric-ready wiring, HERS field verification |
| An attached ADU, a second story, a bump-out | Addition (§150.2(a)) | Mandatory measures plus the addition rules below. No solar requirement. Existing heating can usually be extended |
| A garage or interior conversion to an ADU | Alteration (§150.2(b)) | Only the components you touch have to comply. A replaced water heater may still be gas |
| A junior ADU carved out of the existing house | Alteration | Same as above, and a unit of 500 sq ft or less may use a point-of-use electric water heater |
Get the category wrong at the design stage and the Title 24 report comes back asking for things the budget never carried. Get it right and most of what follows is predictable.
New construction: a heat pump is the default, gas is the detour
The Energy Code gives a designer two ways to comply. The prescriptive path is a checklist: build to the component package in Table 150.1-A for your climate zone and you are done. The performance path models the whole house in approved software and requires it to use no more energy than a reference house built to that same prescriptive package.
In the 2025 cycle the prescriptive package for a newly constructed single-family building, which is what a detached ADU is, specifies a heat pump for space heating and a heat pump water heater (a single 240-volt unit, or a solar water heating system with electric backup). Gas space heating is not an option on the prescriptive checklist at all. That is the change that gets reported online as a gas ban.
A gas furnace or gas water heater is still permitted in a new ADU. It simply forces the project onto the performance path, where the reference house it is measured against has heat pumps. The gas design then has to make up the difference somewhere else: a tighter envelope, better windows, more solar. On a small unit that trade is usually not worth making, which is why most new ADUs in 2026 are electric by arithmetic rather than by mandate.
Two details matter on small units. A newly constructed dwelling of 500 sq ft or less may use an instantaneous electric water heater with point-of-use distribution instead of a tank-type heat pump unit (Exception 1 to §150.1(c)8), which saves a closet. And the solar requirement that applies to a new detached ADU has its own exceptions for small calculated system sizes and shaded roofs; our solar guide walks through them.
Additions: what carries over and what does not
An addition to an existing house has to meet the mandatory measures in §110 and §150.0, plus either the prescriptive addition rules in §150.2(a)1 or a performance model. The exceptions written into §150.2(a) are where the money is, so here they are in plain terms.
- Extending your existing furnace or air conditioner into the addition is allowed. Exception 3 to §150.2(a) says the existing heating and cooling equipment need not comply with Part 6 when it is extended to serve the addition. Whether it has the capacity is a separate question for the mechanical contractor.
- Extended ducts must be sealed and insulated to the current standard. Exception 4: any duct run extended from the existing system, and the existing system it connects to, has to meet §150.2(b)1D. Expect a duct leakage test.
- No solar panels for an addition. Exception 6: the photovoltaic requirement in §150.1(c)14 does not apply to additions.
- A new water heater installed for the addition must be a heat pump water heater. §150.2(a)1D. The alternatives are a NEEA Tier 3 heat pump unit, or for additions of 500 sq ft or less an electric point-of-use heater. A gas water heater for the new wing is not on the list.
- Small additions get relief on roofing and ventilation cooling. Additions of 300 sq ft or less are exempt from the cool-roof requirement (Exception 1); additions of 1,000 sq ft or less are exempt from ventilation cooling (Exception 5).
- A new HVAC system installed for the addition gets field verification. The Commission's HVAC FAQ is explicit: a new system serving an addition must meet all the space conditioning requirements, including refrigerant charge verification on a heat pump. That is a HERS rater visit, and it belongs on the schedule.
Notice the asymmetry. If you replace the house's existing water heater as an alteration, §150.2(b) still lists a natural gas or propane system as a compliant choice. If you add a water heater for the new space, you are in §150.2(a)1D and it is a heat pump. A second-story master suite with its own water heater is the classic case.
What this does to the panel and the budget
A heat pump water heater wants a dedicated 240-volt circuit, a location with enough air volume or a ducting plan, and a condensate drain. A heat pump for space heating wants its own circuit and an outdoor unit location that respects setbacks and noise. On a 1960s house with a 100-amp panel those loads are often the thing that tips the project into a service upgrade, which is a cost that has nothing to do with the square footage. Our guide on panel upgrades covers how to check before demolition. A detached ADU frequently ends up with its own electrical service for other reasons; see the meter question.
The 2025 code also extends electric-ready requirements to additions: wiring and panel capacity roughed in for the electric version of equipment you install today. Which items apply to a given addition is something the Title 24 consultant lists on the compliance form, and it is cheap to do during framing and expensive after drywall.
Climate zone matters for the prescriptive numbers. Most of Santa Clara County sits in Climate Zone 4 and most of San Mateo County in Climate Zone 3, and the Commission's zone finder answers it by address. We do not publish a price for the heat pump package because what it costs depends on the panel, the water heater location and whether the house already has ductwork, and a number without those facts is a guess.
Who decides this on your project
Three people, in order. The Title 24 energy consultant runs the model and produces the Certificate of Compliance (CF1R) that goes in with the permit; that document, not an opinion, is what the plan checker reads. The mechanical and plumbing contractors install to it and sign the installation certificates. A HERS rater verifies the items the code says must be tested.
Our job as the builder is to make sure the gas-or-electric decision is made before the first drawing, not discovered at plan check. On a Project Discovery Call we ask what the house runs on today, what size the panel is, and where a water heater could physically go. Those three answers usually settle the question before anyone pays for a Title 24 report.
Common questions
Did California ban gas appliances in new homes in 2026?
Does a home addition need a heat pump in California?
Is a detached ADU new construction under Title 24?
Which Energy Code applies if my plans were drawn in 2025?
Does an addition need solar panels under the 2025 Energy Code?
Where to go next
Written by the team at Elusive Construction, a licensed California general contractor (B #1112902) building in Santa Clara and San Mateo counties. General information about how residential construction works — not legal advice. For your specific contract, talk to an attorney.
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