ADU Solar Requirements in California: 2026 Energy Code

If you are pricing a backyard cottage in San Diego County this fall, there is a line item that catches a lot of homeowners off guard: solar. Not the panels that may already be on your main house — a new array, sized for the ADU, permitted with the ADU.

California's 2025 Energy Code took effect on January 1, 2026, so every ADU permit application filed this year is reviewed under it. The rules are not complicated. But they hinge on one distinction most people get wrong on the first pass, and getting it right early is worth real money.

The Energy Code cares how your ADU is built, not what you call it

Before anything else, the code sorts your project into one of three buckets. Everything downstream — solar, water heating, insulation — follows from that.

Newly constructed

A detached ADU — a brand-new structure in the backyard that has never been occupied — is treated as a newly constructed single-family building. It carries the full set of new-construction requirements, solar included.

Addition

An attached ADU (one sharing a wall, floor, or ceiling with the house) and a conversion of unconditioned space — the classic garage conversion, or a storage room brought into conditioned space — are treated as additions. They add conditioned floor area, but they do not trigger the new-construction solar requirement.

Alteration

Converting space that is already conditioned — a finished bonus room, a basement, a pool house — is an alteration. So is a junior ADU carved out entirely within the existing house. These are the lightest touch of the three.

That single fork is why two projects with identical square footage can land in very different places. A 700-square-foot garage conversion and a 700-square-foot detached cottage are not the same job in the eyes of the Energy Code. Where the unit sits on your lot is an energy decision as much as a design one, which is why we settle it early in our process rather than leaving it to the drawing stage.

When a detached ADU actually needs solar — and when it does not

Section 150.1(c)14 of the Energy Code requires a solar PV system on newly constructed single-family buildings, and the Energy Commission's ADU guidance for the 2025 code confirms a detached ADU is one of them. But the required system size is calculated, not assumed — and several exceptions can take it to zero.

How the size gets set

The 2025 code sizes the array from solar access roof area (SARA) — the usable, unshaded roof you actually have. SARA is multiplied by 18 watts per square foot on steep-sloped roofs, or 14 watts per square foot on low-sloped roofs. That figure is compared against a second calculation based on climate zone, conditioned floor area, and number of dwelling units, and the smaller of the two governs.

The exceptions that come up on small units

The Commission's single-family solar PV guidance lists five exceptions to that section. Two of them show up constantly on ADU-scale buildings:

• Less than 80 contiguous square feet of solar access roof area. A small footprint with a shaded or awkwardly oriented roof may simply not have qualifying area.
• A required system smaller than 1.8 kWdc. If the calculation lands under that threshold, no array is required.

A third is worth knowing if you were already considering a battery: pairing storage that meets the code's Joint Appendix JA12 requirements, with a minimum cycling capacity of 7.5 kWh, reduces the required PV size by 25 percent.

None of this is a judgment call you make at the kitchen table. Your energy consultant runs the calculation, and the result appears on the compliance forms submitted with your permit set. What you can do is ask for that calculation before the roof pitch, the orientation, and the tree line are locked in.

The panels do not have to sit on the ADU

This is the detail that rescues a lot of projects. The code does not dictate where the modules go. New modules can be added to an existing system elsewhere on the same residential lot — the main house roof, for instance — provided they are sized per code, included in the ADU's permit application, and your utility allows expanding the system.

One important caveat: an existing array, by itself, does not satisfy the requirement. The modules have to be new, and they have to be part of this permit. "We already have solar" is not an exemption.

Solar is not the only 2026 change worth budgeting

The 2025 code cycle moved several other things that show up in a small unit:

• Heat pump water heating is the baseline for new detached ADUs. There is a useful exception: units of 500 square feet or less may use an instantaneous electric water heater with point-of-use distribution — often the cleaner answer in a compact plan anyway.
• Heat pump water heaters need air. The code now specifies ventilation or a minimum room volume where one is installed. In a 600-square-foot cottage, that is a closet-sizing decision your designer needs on day one, not a change order in month four.
• Battery-storage-ready wiring is mandatory for newly constructed single-family units with electrical service over 125 amps, unless a battery is actually installed. It is prep work rather than equipment, but it belongs in the estimate.
• Your existing furnace cannot serve both buildings. Ducts from the main house's system cannot also serve the ADU, so plan on a dedicated mini-split or multi-split — usually the better mechanical answer regardless.

What this means for your budget and your drawings

Two practical takeaways.

Decide detached versus attached with the full picture in front of you. A detached unit buys privacy, a cleaner rental separation, and more flexibility down the road. It also carries new-construction energy requirements. An attached unit or a garage conversion sidesteps the PV trigger entirely. Neither is right in the abstract — it depends on your lot, your goals, and what the two versions actually cost side by side. That comparison is worth running on paper before anyone draws anything, and it is a normal part of how we scope an ADU project.

Get the energy calculation done early. The PV number, the water heater type, and the panel-capacity question all land better in schematic design than in plan check. Working from standardized plan sets helps here, because the energy package has already been worked through for a known building rather than reinvented for a one-off.

Solar on an ADU is rarely the thing that makes or breaks a project. But an unexpected array quoted in month three, on a roof that was never oriented for it, is exactly the kind of surprise that sours an otherwise good build. It is cheap to get ahead of and expensive to discover late.

If you would like to know which bucket your project falls into, and what the energy package will realistically cost, book a free site walk. We will look at your roof, your electrical panel, your setbacks, and your shade, and give you a straight answer before you commit to a direction. As a construction management agency, our job is to coordinate the designer, the energy consultant, and the trades on your behalf — and to make sure the code questions get asked in the right order.

No rush, no pressure. Just a clear picture of what your backyard can actually do.