New unit, new code, and a permit trap that looks like a green light.
Whether a new accessory dwelling unit needs solar depends on how it is classified: a detached ADU is new construction and generally requires a photovoltaic system, while an attached ADU and conversions of existing space are additions or alterations and do not. Where a system is required, the size comes from a compliance report — and it differs unit by unit on the same lot.
Get a free estimateFirst, whether solar is required at all depends on how the unit is classified. The Energy Commission draws the line clearly: a new detached ADU is new construction and generally requires a photovoltaic system under Section 150.1(c)14. An attached ADU is an addition, and solar is not required. Converting a garage or other unconditioned space is likewise an addition. Converting space that is already conditioned is an alteration. Additions and alterations do not carry the new-construction PV requirement.
Where a system is required, the size comes from the project's energy compliance report. Conditioned floor area is a direct input to that calculation — alongside climate zone, dwelling unit count, the code cycle in force and applicable exceptions — but floor area alone does not give you the number, and a rule of thumb based on it is unreliable. Two units on the same lot can carry different minimums; on this Winnetka project they did.
The code cycle is tied to the date your permit application is filed, not the year you got the estimate. A project quoted in one cycle and permitted in the next is assessed under the newer one.
The system
| Item | Detail |
|---|---|
| Location | Winnetka, Los Angeles, CA |
| Ground-floor unit | 975 sq ft — minimum 2.3 kW, installed 6 panels at 2.67 kW |
| Upper unit | 706 sq ft — minimum 2.07 kW, installed 5 panels at 2.225 kW |
| Second property, per unit | 4.895 kW DC / 4.209 kW AC (CEC) |
| Panels | Canadian Solar CS3W-445MB-AG, 445 W |
| Inverters | Enphase IQ7A-72-2-US, 240 V |
| Estimated first-year production | 5,892 kWh per unit |
| Consumption offset | 107% |
Which ADUs actually require solar
Before the sizing question there is a classification question, and getting it wrong in either direction is expensive. The Energy Commission's guidance for the 2025 code sorts ADU projects like this:
| Project type | Classified as | PV required? |
|---|---|---|
| New detached ADU | New construction | Generally yes, subject to exceptions |
| Attached ADU | Addition | No |
| Garage or unconditioned space conversion | Addition | No |
| Conversion of already-conditioned space | Alteration | No |
An existing array on the property does not by itself satisfy the requirement for a new detached ADU — though modules can be added to it on the same site rather than building a separate system.
Two units, two different minimums
On one Winnetka property the ground-floor unit at 975 square feet and the upper unit at 706 square feet each came back with their own required size. We sized both above the minimum rather than exactly on it, which leaves the compliance margin intact if anything shifts during construction:
- ground-floor unit — minimum 2.3 kW, installed 6 panels at 2.67 kW
- upper unit — minimum 2.07 kW, installed 5 panels at 2.225 kW
On a second property the two units differed again — one created through a lot-split pathway, one an accessory dwelling unit.
The rule builders are most often told wrong
The lot-split legislation does not itself impose a solar requirement. It creates a route to build additional primary units. The energy obligation comes from the energy code and how the building is classified — not from the statute that permitted the unit to exist.
Builders are told otherwise often enough that it is worth stating plainly.
The permitting trap
This is the one that costs builders weeks.
A project can appear in the city's general permit records and still not be available in the express permitting system. They are different systems. Seeing the project listed in one does not mean you can file in the other.
On the second unit of one property we could not pull the permit until the address itself appeared in the express system. We watched for it and filed the moment it did. A builder who treats the general listing as a green light waits for a rejection that never arrives — because the application was never accepted.
Why builders can schedule around us
The arrangement is deliberately simple: contract, a small deposit against permit costs, and we are ready to install the moment the roof is ready. Solar on new construction sits at the end of a long chain of other trades, and a solar contractor who cannot move on short notice becomes the reason a certificate of occupancy slips.
The pace shows in the record. The first quote went out the same day as the call that started the project. When the builder was ready to proceed, the contract was drawn, sent and signed within three days. Permits were filed the moment each address became filable — on one unit we were watching the express system daily and submitted the day it opened.
Where a builder runs two projects with us at once, we price them together.
He came back with the next set of plans
This builder came back with new addresses.
A homeowner installs solar once and rates the experience. A builder works with contractors continuously, watches all of them closely, and can replace any of them without explanation. When a builder sends the next set of plans, that is a different kind of evidence than a review.
More than a year after the first unit, the plans are still arriving — the most recent request for a quote on a new project came this August. Every unit we have sized for this builder has cleared its energy compliance, none has held up a certificate of occupancy, and the working relationship has outlasted several projects.
What we did
- Sized each unit from its own compliance figures rather than applying one number to the lot
- Specified above the minimum on both units to preserve the compliance margin
- Separated the lot-split pathway from the energy obligation for the builder
- Tracked the express permitting system until the second address became filable
- Structured the deposit against permit costs so installation could follow the roof closely
- Priced two concurrent projects together
Does this match your situation?
If any of these describe where you are, this page is about your problem.
You are building an ADU or infill unit and need to know whether solar is required
You have been given a system size based on square footage alone
You have two units on one lot and were told they need the same system
Your project shows in the city's permit records but you cannot file for an express permit
You were quoted in one year and are permitting in the next
Your solar contractor cannot commit to moving when the roof is ready
What to check on your own project
Every property is different. These are the things worth confirming for yours.
- Required system size is project-specific. Nothing here establishes what your project needs — only your compliance documentation does.
- Exceptions exist within the energy code, and whether one applies depends on the project.
- The code cycle applying to any given project depends on its permit application date.
Check the numbers yourself
We analyzed 433 recent Google reviews across seven Los Angeles solar installers — our own included — and published the dataset so anyone can recompute the ranking. Cali Energy came out with the lowest adjusted complaint rate in the group, and we say plainly where that finding is and is not statistically separated.
Other situations we handle
A different problem each time.
Frequently asked
Does a new ADU in California require solar?
Does my existing solar cover the requirement for a new detached ADU?
How is the minimum solar size for a new unit calculated?
Does SB 9 require solar on the new units it allows?
Why can't I file for an express permit when the city already shows my project?
Which energy code cycle applies to my project?
Related reading
Sources & methodology
Figures on this page come from the primary sources below and, where noted, from Cali Energy calculations using the stated assumptions. Rates, incentives, and program terms change; each was verified September 2, 2026.
Tell us what you are dealing with
Send us what you have — a bill, someone else's quote, a permit correction, or just the address. We will tell you what the project actually involves before anyone signs anything.
Get a free estimatePrepared by Cali Energy, September 2, 2026. This article is for general educational purposes only and is not legal, tax, financial, engineering, or utility advice. Rates, incentives, codes, permit requirements, equipment specifications, prices, and program terms may change; figures and timelines are estimates, not guarantees. Confirm current requirements with the applicable utility, AHJ, program administrator, manufacturer, or a licensed professional. See our Content Disclaimer. Cali Energy · 19201 Parthenia St, Unit E, Northridge, CA 91324 · CSLB #1032379 (B, C-10, C-39)