Do You Need Solar for a New ADU in Southern California?

If you’re building a new ADU in Southern California, one of the first questions you’ll probably ask is:

“Do I have to add solar?”

And the honest answer is: Most of the time, yes — but not always in the way you think.

Solar requirements in California can feel confusing because they depend on how your ADU is built, whether it’s new construction vs. a conversion, and what the local building department requires based on the latest California Energy Code.

In this guide, we’ll break it down clearly so you know what to expect before your project starts.

Why California Requires Solar on New Construction

California’s solar rules come from the state’s energy standards (Title 24), which are designed to:

• Reduce greenhouse gas emissions
• Lower long-term utility costs
• Encourage cleaner, energy-efficient housing
• Support the state’s goal of carbon neutrality

In simple terms: if you build new living space, California wants it to produce some of its own energy.

Does Every ADU in Southern California Require Solar?

Not every ADU requires solar, but most detached new builds do.

Here’s the easiest breakdown:

✅ New Construction ADU (Detached or New Addition)

Yes — solar is typically required.

This includes:

• Detached ADUs
• New builds behind the home
• ADU additions built from scratch
  
  

⚠️ Garage Conversion to ADU

Not always.

Many conversions do not trigger the same solar requirement because you’re converting existing conditioned space rather than building brand-new square footage.

However, this can vary depending on:

• How much structural work is being done
• Whether the conversion includes a new addition
• The city’s interpretation of Title 24
  
  

⚠️ Junior ADU (JADU)

Usually no.

JADUs are typically interior conversions, so they usually do not require solar.

What If Your Property Already Has Solar?

Good news: you might not need to add a full new system.

If your home already has solar, the city may allow:

• A system expansion
• A smaller additional system
• A new system sized only for the ADU load

It depends on:

• Existing system size
• Current electrical panel capacity
• Energy calculations for the ADU
• Local utility requirements

How Much Solar Is Required for an ADU?

This is where it gets tricky — because it’s not one-size-fits-all.

Your required solar size depends on:

• ADU square footage
• Energy efficiency specs
• HVAC type (mini-split, heat pump, etc.)
• Water heater type
• Appliance load
• Window performance
• Insulation and framing details

The final solar requirement is calculated through Title 24 energy modeling, and the solar sizing typically happens during plan preparation.

What If Your ADU Roof Doesn’t Work for Solar?

This is actually common — especially with:

• Narrow lots
• Shaded backyards
• Two-story structures
• Rooflines with limited south-facing slope
• Large trees or neighboring buildings

If your roof doesn’t work, there may be alternatives such as:

• Installing solar on the main house roof
• Using a different roof plane
• Adjusting roof design slightly to create solar-ready space
• Solar-ready compliance strategies (depending on scope)

Do You Need Battery Storage Too?

This is another question we get constantly.

In most cases:

No — battery storage is not required.

But some homeowners choose to add a battery for:

• Backup power
• Long-term savings
• Future-proofing
• EV charging support

Battery storage can also be helpful when the ADU will be used as a rental, since it can reduce peak-time energy costs.

What About Solar for pre-designed ADUs?

If you’re considering a prefab or modular unit, solar still usually applies if the unit is considered new construction.

That said, prefab solar requirements depend on:

• The manufacturer’s specs
• The local jurisdiction
• How the unit is installed and permitted

This is why it’s important to work with a builder who understands both:

• The build side
• The permitting side
  
  

Solar Adds Cost — But It’s Not Always as Bad as People Think

Solar is an added line item, but it’s also one of the most predictable costs in an ADU project compared to:

• Sewer connection upgrades
• Electrical panel upgrades
• Water meter requirements
• Utility trenching
• Unexpected grading

Many homeowners are surprised to learn that solar can often be sized efficiently for an ADU — especially when paired with energy-efficient systems.

How We Handle Solar & Permitting for ADUs in Southern California

At ADUs by Avatar, we handle the full process — including:

• Title 24 energy compliance
• Solar requirements and sizing
• Permit submittals and plan corrections
• Coordination with solar installers
• Electrical panel upgrades (if needed)
• Utility coordination for new units

We make sure the solar portion of your ADU doesn’t become a surprise cost halfway through your build.

Final Thoughts: Solar Is Part of the New Normal for ADUs

If you’re planning a new ADU build in Southern California, solar is something you should expect from day one.

The key is making sure your project is designed correctly so solar:

• Fits the roofline
• Meets code
• Passes plan check
• Supports long-term energy savings

And most importantly — so it doesn’t slow down your permit timeline.

Sources

• California Energy Commission – Solar Requirements for New Construction
  California’s official energy code guidance on solar for new buildings
  https://www.energy.ca.gov/programs-and-topics/programs/building-energy-efficiency-standards/energy-code-support-center/solar