California’s ADU Solar Requirements: Does My ADU Need Solar Panels?

As California continues to lead the way in energy-efficient housing, homeowners looking to build an Accessory Dwelling Unit (ADU) must navigate a growing list of sustainability requirements — including solar panel mandates. While Title 24 of the California Energy Code has made solar systems a standard feature for many new homes, the rules surrounding ADUs are more nuanced. Whether you're planning a detached backyard unit, converting your garage, or adding space to your existing home, it's important to know if your ADU will be required to include solar panels. The answer depends on several factors, including the type of ADU, its size, and how it’s constructed. At SFbayADU, we specialize in helping homeowners in the San Francisco Bay Area understand and comply with local and state building codes. In this guide, we’ll break down the latest solar requirements for all types of ADUs so you can plan your project with clarity and confidence.

2. Overview of California’s Solar Mandate

California’s solar requirement stems from Title 24, Part 6 – the building energy efficiency standards set by the California Energy Commission, which require all new low-rise residential construction—including ADUs—to include a solar photovoltaic (PV) system unless an exemption applies. This solar mandate, sometimes called the California Solar Mandate, took effect for new homes starting in 2020, with comprehensive inclusion of ADUs under the 2022 code revision, effective January 1, 2023. Title 24 requires that the installed PV system must at minimum meet the capacity calculated using Equation 150.1‑C, which factors conditioned floor area and climate zone—unless limited solar access or other code exceptions apply (https://www.energy.ca.gov/programs-and-topics/programs/building-energy-efficiency-standards/energy-code-support-center/solar). In practical terms, here’s what that means: New detached ADUs are classified as “new construction” and generally must include solar PV, sized per Title 24 formulas or limited by available roof area. Conversions or additions—such as garage conversions, internal conversions (e.g., basement or attic), attached ADUs, and Junior ADUs—are treated as alterations/additions. For these, the solar requirement does not apply. If the calculated PV size for a new ADU is below approximately 1.8 kW, that unit qualifies for a small-ADU exemption, meaning no solar installation is legally required. Keep in mind that solar-ready provisions, which require roof reinforcement and conduit installation for future PV systems, are only required for large new subdivisions, not for ADUs on existing lots that lack PV systems (https://www.energy.ca.gov/programs-and-topics/programs/building-energy-efficiency-standards/energy-code-support-center-6/2022). Overall, Title 24’s solar mandate represents California’s commitment to carbon-neutral building practices and significantly shapes ADU planning—making it essential for homeowners and builders to understand how the rules specifically apply to different ADU scenarios. 3. Detached ADUs – When Solar Panels Are Required A detached ADU is a new, standalone dwelling unit constructed separately from the main house on the same lot. Title 24 Solar Requirement Under California Energy Code, Title 24 (2022 standards), newly constructed, detached ADUs are considered new single-family residences and are therefore generally required to include a solar photovoltaic (PV) system as part of the permit application process. How Solar Size Is Calculated Title 24 requires that the PV system be sized using Equation 150.1‑C, which takes into account the unit’s: Conditioned floor area (CFA) Climate zone Number of dwelling units on the site If your existing home already has solar panels, you may still be compliant by adding enough PV modules to the existing array to meet the ADU’s minimum requirement, as long as they are installed on the same lot and included in the permit application—even if powering the primary house. Key Exemptions Title 24 includes two key exceptions that may exempt a detached ADU from having to install solar panels: Minimum Required PV Size < 1.8 kW‑dc. If the calculated PV requirement (per Equation 150.1‑C) is under 1.8 kW, the ADU qualifies for the small‑ADU exemption, and no solar installation is mandated. In practice, this often means that detached ADUs under approximately 600–700 sq ft (depending on climate zone) are exempt. Limited Solar Access Roof Area (SARA) If the Solar Access Roof Area is less than 80 contiguous square feet (due to shading or orientation constraints), the solar requirement is waived for that unit. Battery-Ready Requirement Even when solar is required, all newly built detached ADUs must also meet ESS‑ready (energy storage system readiness) standards under Title 24 Section 150.0(s). This requires wiring and provisions for future battery storage, though the batteries themselves are optional. In short, for new stick‑built detached ADUs, solar panels are typically required, but you may qualify for an exemption if the unit is small enough or has limited roof solar access. Let me know if you’d like examples of PV sizing for specific square footage or climate zones. Summary Table ADU Type Solar Required? Exemptions Detached, new-built ✅ Yes, unless exempt < 1.8 kW requirement;< 80 sq ft SARA area In short, for new stick‑built detached ADUs, solar panels are typically required, but you may qualify for an exemption if the unit is small enough or has limited roof solar access. 4. Attached ADUs – No Solar Required An attached ADU is an accessory dwelling unit that shares at least one wall with the primary residence. It may be added to the side, rear, or even above a portion of the existing home, but it remains physically connected to the main structure. Solar Panel Requirement Under California’s Title 24 Energy Code, attached ADUs are treated as additions, not new stand-alone constructions. As a result, they are not subject to the solar photovoltaic (PV) system requirement that applies to new single-family homes or detached ADUs. Why They’re Exempt The solar mandate in Title 24 is intended for new residential buildings, which does not include additions or alterations to existing structures. Because an attached ADU is legally categorized as an addition to the main home, the project is exempt from mandatory solar installation. Even if the attached ADU includes a new roof or significantly expands the home's footprint, it still falls under the "addition" category — and therefore does not trigger a requirement to install solar panels. Advantages of Attached ADUs Cost savings: No need to invest in solar equipment or related wiring. Simplified permitting: Less complexity during the energy code compliance review. Flexibility: Ideal for homeowners looking to build more affordably without sacrificing square footage or amenities. While solar panels are not required for attached ADUs, other energy efficiency measures under Title 24 (such as insulation, high-efficiency windows, and HVAC systems) still apply to the new space. In summary, if you're building an attached ADU, you won’t need to install solar panels under current California regulations — making this a practical option for homeowners who want to expand their living space without the added cost of solar compliance. 5. Garage Conversions – Also Exempt A garage conversion ADU refers to transforming an existing garage—or similar structure—into a living unit. This often includes detached garages or garages attached to the primary residence, provided they were previously a non-residential or unconditioned space. Solar Panel Requirement Under California’s Title 24 energy code, converting an existing garage into an ADU is considered an addition or alteration, not new construction. This classification means that no solar photovoltaic (PV) system is required for such conversion projects—including both attached and detached garage ADUs. The California Department of Housing and Community Development (HCD) affirms that ADUs converted from existing structures—such as garages—are not subject to the state’s solar mandate, provided the conversion does not involve demolition and rebuilding from scratch. Why Garage Conversions Are Exempt Title 24 differentiates between new construction (which triggers solar requirements) and alterations/conversions (which do not). Since garage conversions re-purpose existing building envelopes, they fall into the latter category. Even a fully detached garage, once converted without demolition, counts as an alteration and stays exempt. Important Consideration If the existing garage structure is completely demolished and a brand-new ADU is built in its place, Title 24 treats this as new construction, and it would trigger the solar requirement. Be sure to clarify project scope with your permit authority during early planning to avoid misunderstandings that could unexpectedly trigger solar compliance. Other Compliance Requirements Although solar is not required, garage conversion ADUs must still comply with other energy efficiency standards under Title 24, such as: Installation of insulation and high-efficiency windows; Upgraded HVAC or water heating systems where applicable; Efficient lighting, ventilation, and envelope upgrades as needed. Garage conversion ADUs—whether attached or detached—are exempt from solar panel requirements under current California regulations, since they are classified as alterations rather than new builds. However, standard energy code requirements around insulation, lighting, appliances, and ventilation still apply. 6. Internal ADUs and Junior ADUs (JADUs) – Always Exempt An internal ADU is created by converting existing habitable space within the primary residence—such as a basement, attic, or unused room—into a separate living unit. A Junior ADU (JADU) is a specific subtype of internal ADU that is 500 square feet or less and must be contained entirely within the existing walls of a single-family home, often sharing a bathroom with the main dwelling. Solar Panel Requirement According to California’s Title 24 Energy Code, internal ADUs and JADUs are completely exempt from solar photovoltaic (PV) requirements. These units are considered additions or alterations to existing structures, and not new construction, which is the key trigger for the state’s solar mandate. The California Energy Commission and the Department of Housing and Community Development (HCD) both clarify that solar panels are not required for: ADUs created entirely within an existing home’s footprint. JADUs as defined by state law (≤ 500 sq ft, within an existing home). Why They're Exempt The Title 24 solar mandate applies only to newly constructed, detached residential buildings. Internal ADUs and JADUs: Do not increase the building’s footprint. Do not alter the roof area or solar exposure. Do not require new standalone utility service. Because these units utilize existing conditioned or semi-conditioned space, they are treated as minor modifications rather than structural additions—keeping them fully outside the scope of solar requirements. Benefits of Internal ADUs and JADUs Cost savings: No need to install solar panels or battery-ready electrical systems. Faster approvals: Fewer Title 24 compliance hurdles. Minimal disruption: No exterior construction or major site work required. Other Considerations Even though solar panels aren’t required, these units must still meet applicable building codes related to: Insulation and air sealing (if walls or ceilings are modified), Lighting and ventilation, Fire and sound separation (depending on configuration). If you’re converting space within your existing home or adding a JADU, you’re automatically exempt from California’s solar panel requirements. These are among the most cost-effective and code-friendly types of ADUs, making them a smart option for homeowners seeking to add value and functionality without triggering major construction rules. 7. Special Cases and Exemptions While California’s Title 24 solar mandate clearly applies to most newly constructed detached ADUs, there are several important special cases and exemptions that may relieve homeowners from having to install solar panels. Understanding these exceptions can help you plan your ADU project more efficiently and cost-effectively. 1. Small ADU Exemption (<1.8 kW PV Requirement) If the calculated size of the required solar system is less than 1.8 kW (DC), your ADU is exempt from solar installation. This threshold is based on Equation 150.1-C in the Energy Code, which factors in square footage and climate zone. In many cases, this means that ADUs smaller than approximately 600–700 sq ft (depending on location) will not be required to install solar panels. This exemption was introduced in the 2022 Energy Code, effective January 1, 2023. 2. Limited Solar Access (Shaded or Small Roof) If your ADU has a Solar Access Roof Area (SARA) of less than 80 square feet, due to permanent shading or lack of roof space: The ADU is exempt from the solar mandate. Shading can be caused by nearby trees, other buildings, or roof obstructions like chimneys or skylights. Orientation and tilt limitations may also impact solar feasibility. 3. Manufactured and Modular ADUs Prefabricated ADUs, including manufactured homes, mobile homes, and modular units, are regulated under different federal or state standards: These units are not subject to California’s Title 24 solar PV requirements. Even if installed on a permanent foundation, they are exempt because they are not site-built dwellings. 4. Additions to Homes Without Solar If your main residence does not have solar, and you're building an attached ADU or converting part of the home, you are not required to add a system just for the ADU. Title 24 does not impose retroactive solar mandates on existing homes or additions to them. 5. Non-Demolition Conversions of Existing Structures If you are not demolishing the existing structure, but simply converting a garage, attic, or shed into an ADU: The project is classified as an alteration, not new construction. Therefore, no solar installation is required, even if the structure is detached from the main home. 6. Local Ordinances and Utility Restrictions In rare cases, local jurisdictions or utility providers may: Have stricter or more lenient requirements than the state. Limit new grid-tied solar connections due to infrastructure capacity. Offer incentives or exemptions for certain property types or zones. It’s always best to check with your local building department or work with an experienced ADU builder like SFbayADU to determine your exact compliance path. While California is committed to solar energy, many ADUs qualify for exemptions based on size, shading, construction type, or project scope. At SFbayADU, we help clients evaluate every option to avoid unnecessary costs while staying fully compliant with building and energy codes. Reach out to our team for a personalized evaluation of your ADU's solar requirements. 8. How SFbayADU Helps Homeowners Navigate Compliance At SFbayADU, we understand that navigating California’s complex building and energy codes—especially when it comes to solar requirements—can feel overwhelming. That’s why we offer full-service guidance to ensure your ADU project complies with all state and local regulations, without unnecessary delays or costs. Here’s how we help: Comprehensive Project Evaluation From the initial consultation, we assess your property, project type, and ADU design to determine whether your unit falls under solar requirements or qualifies for an exemption. We take into account factors such as: ADU type (detached, attached, conversion, or JADU) Square footage and climate zone Existing roof space and shading Whether it’s new construction or an alteration Title 24 Compliance Reports We prepare and submit all necessary Title 24 energy documentation, including: Energy modeling calculations Solar sizing reports (if applicable) Documentation of any exemptions (e.g., <1.8 kW, limited roof access) This ensures your permit application is accurate, complete, and code-compliant from the start. Coordination With Solar Designers (If Needed) If your detached ADU requires solar panels, we can coordinate with trusted solar system designers and licensed installers. Our team helps you: Evaluate the most cost-effective PV system options Explore whether the ADU can share a system with the main home Ensure your electrical plans include required battery-ready infrastructure Local Permit Support We stay up-to-date with local building departments’ interpretations of state code, which can vary by city or county. We handle the back-and-forth communication to clarify requirements, respond to plan checker questions, and prevent costly re-submissions. Strategic Planning for Cost Efficiency Our experience allows us to recommend creative, compliant solutions—such as: Modifying square footage or orientation to avoid unnecessary solar costs Retaining existing structure elements to qualify as a “conversion” Advising when prefab or modular ADUs may be more cost-effective and solar-exempt End-to-End ADU Project Management From design and planning through construction, our team manages the entire process. Whether your project requires solar or qualifies for an exemption, we ensure that energy compliance is never a barrier to progress. Bottom line: At SFbayADU, we make sure your ADU meets all solar and energy code requirements without surprises. Our goal is to deliver a seamless, efficient, and fully compliant building experience—so you can focus on enjoying the new space, not stressing over regulations. 9. Conclusion California’s solar requirements for ADUs are rooted in the state’s broader commitment to sustainability—but that doesn’t mean every ADU project will require solar panels. Whether or not your ADU must include a photovoltaic (PV) system depends on the type of ADU, its size, roof conditions, and how it’s built. Detached new-construction ADUs typically require solar, unless they qualify for exemptions based on size or roof shading. Attached ADUs, garage conversions, internal ADUs, and Junior ADUs (JADUs) are generally exempt from the solar mandate. Special cases—such as manufactured ADUs and conversions that retain the original structure—also avoid solar requirements under current California Energy Code (Title 24). At SFbayADU, we guide homeowners through every step of the process, helping you determine exactly what’s required for your specific project—and just as importantly, what’s not. We handle compliance, documentation, and code interpretation so you don’t have to.