Solar on a Detached ADU
Build a new, freestanding ADU in Del Mar and the state Energy Code expects new solar panels with it. Convert a garage or add on to the house and it doesn't. Here's how the rule sorts projects, and the flexibility most owners don't know about.
Published Nov 11, 2026
Solar is one of those ADU questions with a clean answer and a lot of fine print. The clean answer: a newly built, detached ADU needs new solar panels under California's Energy Code, unless an exception applies. Most other ADU types don't.
The fine print is where Del Mar owners can save themselves trouble. The panels don't have to sit on the ADU.
How the Energy Code sorts ADUs
The California Energy Commission's 2025 ADU FAQs put every ADU into one of three buckets. The bucket decides which rules apply.
| Project | Energy Code category | Solar PV required? |
|---|---|---|
| New, freestanding ADU | Newly constructed building | Yes, unless an exception applies |
| Attached ADU sharing a wall, floor or ceiling | Addition | No |
| Garage or storage space converted to living space | Addition | No |
| Finished basement, pool house or studio turned into an ADU | Alteration | No |
| Manufactured home with a HUD label | Not covered by the Energy Code | No |
The state housing department's ADU Handbook says the same thing in plainer terms. ADUs built inside existing space, or as additions to existing homes, aren't subject to the solar requirement. That includes a detached building converted from non-residential to residential space.
A factory-built home is different from a manufactured home. The CEC says factory-built ADUs must meet Title 24, including the Energy Code.
The flexibility: panels on the main house
Here's the part that surprises people. The CEC says new solar modules for a detached ADU don't have to be installed on the ADU.
According to the FAQs, new modules can be added to an existing system on the lot to meet the requirement, as long as they're part of the ADU's permit application, sized under Section 150.1(c)14 and compliant with the rest of the building code. The Energy Code doesn't say whether they must serve the ADU's meter or the main house's meter. Expansion of the existing system has to be allowed by the utility.
Why that matters on a Del Mar lot:
- A small ADU roof may be shaded by trees or the main house.
- The main house may already have a good south or west roof face.
- Design review and neighbor views may favor a clean ADU roofline.
- One combined array can be simpler to maintain.
What doesn't count
Two limits keep this honest.
An existing system alone won't do it. The CEC says a newly constructed detached ADU must have newly installed PV unless an exception applies. You can expand an old array. You can't point at it and call it done.
The new panels must be in the ADU permit. They're part of the ADU's compliance, not a separate later project.
Exceptions exist
Both the HCD handbook and the CEC note that exceptions apply to the requirement. They live in the Energy Code's single-family solar section and turn on site conditions. Your Title 24 energy consultant runs the compliance model and will flag whether one fits your lot. Don't assume an exception until it's on paper.
Two look-alike rules
Two related rules often get mixed up with the PV requirement.
Solar-ready. The CEC says solar-ready rules apply to new single-family homes in subdivisions of ten or more. A new ADU on an existing lot isn't covered.
Storage wiring (Section 150.0(s)). A new detached ADU must meet the battery energy storage system ready rules in Section 150.0(s), per the CEC, unless the ADU's electric service is 125 amps or less. Installing a complete battery system also satisfies it. This affects panel and subpanel choices, so settle it early. Our utilities page covers service planning.
Gas appliances still need future wiring
If you plan gas appliances in a new detached ADU, the CEC says it still has to meet the electric-ready requirements. Practically, that means wiring and space for future electric equipment. It's another reason to plan the electrical design and the solar design together.
Where solar shows up in the paperwork
On a detached ADU, solar isn't a separate errand. It runs through the whole permit package.
- The energy compliance report. Your Title 24 consultant documents how the project meets the Energy Code, including the PV requirement or the exception relied on. The required array size comes from Section 150.1(c)14, and the consultant does that math.
- The plan set. The drawings show where the new modules go, whether on the ADU or another roof on the lot, along with the electrical path back to the panel.
- The utility side. If you're expanding an existing array, the CEC's answer assumes the utility allows the expansion. Ask about the interconnection process early so it doesn't hold up the end of the job.
- Inspection. Because the panels are part of the ADU permit, expect them to be installed and inspected as part of that project rather than added later.
Planning these pieces together keeps the roof framing, conduit routes and electrical design from being redone at the last minute.
Design and appearance
Panels are part of how a building looks. State law lets cities apply objective standards to ADUs, including architectural review (Government Code section 66314(b)(1)). Ask your designer how panel placement fits Del Mar's design rules and any coastal permit conditions. See ADU design and our detached ADU page for the bigger picture.
Five things to settle with your designer
- Is this ADU a newly constructed building, an addition or an alteration under the Energy Code?
- If PV is required, which roof on the lot performs best?
- Does the existing system allow expansion with the utility?
- Does the ADU's service size trigger battery-ready rules?
- How do panels fit the design and coastal review?
A note on conversions
If a detached solar requirement sounds like a deal-breaker, compare options honestly. A garage conversion or attached ADU is treated as an addition and doesn't carry the PV requirement. Those choices have their own trade-offs in size, layout and privacy, so solar shouldn't be the only factor.
Let's look at your lot
We work with verified contractors who build detached and converted ADUs in Del Mar and coordinate the energy design with the plans. Request a Free Quote to talk through your lot.
Sources (accessed Nov 11, 2026): 2025 Energy Code Accessory Dwelling Units (ADU) FAQs (California Energy Commission) · Accessory Dwelling Unit Handbook (2026 update) (California Department of Housing and Community Development) · Government Code section 66314 (California Legislative Information)