ADU solar requirement and Title 24: does an ADU need solar?

The ADU energy code in 2026

California's energy code is Part 6 of Title 24, which is why builders call the energy paperwork “Title 24.” The current version, the 2025 Building Energy Efficiency Standards, was adopted by the Energy Commission in September 2024 and applies to buildings whose permit applications are filed on or after January 1, 2026. The Commission announced in January 2026 that the update is in force, with a push toward heat pumps, electric-ready homes and better ventilation.

For ADUs, the key question is whether the unit is a new building. The Energy Commission's ADU FAQ says a detached ADU with no shared walls, ceilings or floors is a newly constructed building and must meet the single-family new-construction rules. Attached ADUs are treated as additions, which carry lighter requirements.

ADU solar requirement for detached units

The Energy Commission says newly constructed detached ADUs must have newly installed solar PV to meet the prescriptive requirements of Section 150.1(c)14, unless an exception applies. The same FAQ answers the questions homeowners ask most:

• Where the panels go. The code doesn't say. They can go on the ADU roof, the main house roof or elsewhere on the lot.
• Existing panels don't count. Solar already on your house doesn't satisfy the rule by itself. Adding new modules to the existing system does, if they're on the same lot, your utility allows the expansion and the new modules are part of the ADU's permit.
• Which meter. The code doesn't specify whether the panels serve the ADU's meter or the main house's.
• Solar-ready. The separate solar-ready rules don't apply to a new ADU built on an existing lot.

In the City of San Diego, the city's ADU bulletin notes that the solar requirement applies to newly built detached ADUs that are not manufactured homes. That's one cost advantage of a manufactured prefab ADU.

Does an ADU need solar? Exceptions and system size

The required system size is the smaller of two numbers, according to the Energy Commission:

• The formula. Equation 150.1-C, which multiplies conditioned floor area by a factor and adds an amount per dwelling unit, using values that vary by climate zone. A bigger ADU needs a bigger system.
• The roof limit. The solar access roof area (SARA), meaning the unshaded roof that can hold panels, multiplied by 18 watts per square foot for steep roofs or 14 watts for low-slope roofs.

Then come the exceptions. The Commission lists five for single-family buildings; the ones that matter most for ADUs:

The 1.8 kWdc floor is why many small ADUs skip solar. Snap ADU, a San Diego builder, says units under about 620 square feet may fall below it in greater San Diego, depending on climate zone. Treat that as a rough guide: your Title 24 consultant runs the actual number for your plan and location. Our small ADU guide covers the other trade-offs of building small.

Attached ADUs, conversions and junior ADUs

Attached ADUs. Because attached ADUs are additions, solar PV isn't required. That's a real saving; our attached vs detached ADU guide weighs it against the fire-wall and structural costs of attaching.

Conversions. A garage conversion ADU or a basement or attic ADU reuses an existing structure rather than putting up a new building, so it isn't the “newly constructed detached ADU” the solar rule targets. The energy code still applies to the work, as an alteration or addition. Confirm the classification with your building department before you budget.

Junior ADUs. The City of San Diego's ADU bulletin says a JADU doesn't need a solar PV system. See our junior ADU guide.

Community solar instead of rooftop panels

The 2025 code lets the on-site solar requirement be offset by a community shared solar system the Energy Commission has approved under Section 10-115. Approval is program by program, and each one serves a specific utility's territory. In 2026 the Commission approved Plumas-Sierra Rural Electric Cooperative's program for new single-family buildings in parts of Lassen, Plumas and Sierra counties.

To qualify, a program has to deliver the same or better energy performance as on-site panels, cut the participant's bills by more than it costs to join, and keep each building enrolled for at least 20 years. We didn't find an approved program serving San Diego homes; if you build elsewhere, ask your building department whether one covers your address.

Other Title 24 ADU rules

Solar is the headline, but a new detached ADU picks up the rest of the single-family new-construction rules too. From the Energy Commission's ADU guidance:

• Ventilation. Every newly constructed dwelling unit must meet the mandatory indoor air quality ventilation rules, whatever its size.
• Battery-ready. Applies to ADUs, but not when the ADU's electric service is 125 amps or less.
• Small-unit water heating. A new unit of 500 square feet or less can use an electric water heater with point-of-use distribution.
• Heat pumps. The 2025 code makes heat pumps the prescriptive baseline for space and water heating in single-family homes.
• Electric-ready. A gas water heater or furnace requires a dedicated circuit and reserved panel space for a future heat pump.

Your designer proves all of this in the Title 24 energy calculations, which San Diego requires with the permit submittal; see how to get an ADU permit in San Diego. The electrical side, including panels and meters, is in our ADU utilities guide.

Designing a detached ADU for solar

Because the required system is capped by usable roof area, the roof design decides a lot. A few choices at the design stage make solar cheaper or, in some cases, unnecessary.

Roof shape and orientation

For steep roofs, the solar access roof area leaves out roof faces pointing between 300 and 90 degrees from true north, roughly the northwest-to-east side. A simple roof with one large south- or west-facing plane gives the installer room; a roof broken up by dormers, vents and skylights gives less. Low-slope roofs are counted at a lower wattage per square foot than steep ones.

Main house or ADU roof

Since the code doesn't fix the location, compare both roofs. Putting new modules on the house can make sense if it has a better orientation or an existing system your utility will let you expand. Putting them on the ADU keeps the ADU's permit self-contained and the wiring short. Either way the new modules go on the ADU permit.

Panels, meters and batteries

Solar ties into your electrical plan, so decide it alongside the meter and panel questions in our ADU utilities guide. A battery isn't required for the ADU, but the battery-ready rules can apply depending on the electric service size. If you're still choosing a design team, our ADU architect guide covers who prepares the Title 24 calculations.

Timing

The energy calculations are part of the permit set, so the solar size is fixed before you apply. Panels are usually installed late in the build, after the roof is on, and are inspected before the final sign-off. See how long it takes to build an ADU for the full schedule.

What ADU solar costs

These are builder estimates, not quotes; system size, roof type and whether you expand an existing array all move the price. Snap ADU also notes that ADU solar under SDG&E's current net billing rules earns less for power sent back to the grid.

Ways to lower the solar cost

• Expand an existing array. If your house already has solar and your utility allows the expansion, adding modules to that system can cost less than a new standalone system.
• Size from the real numbers. The requirement is the smaller of the formula and the roof limit, so have the Title 24 consultant calculate both rather than accepting a round-number system in a bid.
• Build smaller or attached. A unit that falls under the 1.8 kWdc minimum, or an attached ADU, needs no panels at all. Weigh that against the living space you give up.
• Get solar priced as its own line. Then you can compare it with a quote from a separate solar installer.

Don't count on the old 30% federal credit: the residential clean energy credit doesn't apply to systems placed in service after December 31, 2025. See ADU grants in California for what's still available, and what an ADU costs in San Diego for the full budget. When you compare bids from verified ADU builders in San Diego, check whether solar is included or listed as an extra.

Common questions