Do I need solar on my new ADU?

You may have heard that the state of California is making an effort to move all new construction in the direction of sustainability and renewable energy. This has some important considerations for new accessory dwelling unit (ADU) builds that all curious homeowners should be aware of. Read on to learn when and why you need to install solar panels onto your new California ADU. Enjoy a comprehensive guide to all things solar for your guest house.

Do I need to install solar panels for my Accessory Dwelling Unit?

Yes. As mentioned, California state law requires that any newly-built accessory dwelling unit must have solar panels. The California Energy Commission (CEC) states that panels can be installed on the accessory dwelling unit or the primary unit. Adding solar panels onto new construction is common practice for California contractors, as installation is quick and easy when you are already in the process of building.

If you already have solar panels on your primary dwelling, you may add new photovoltaic (PV) modules to the existing system to meet energy code requirements for the ADU. The new PV modules must be part of the ADU’s permit application, sized per the Energy Code, and comply with other parts of the building code as applicable.

If you plan to install solar panels or expand the existing solar panel system on the primary residence to satisfy the energy requirements of the ADU, it may be the case that the new solar modules require an upgrade to your existing electrical service. In this case, it is imperative that the new solar panels or solar expansion is installed after the service upgrade is complete. Electrical service upgrades should always be performed by the ADU contractor to ensure that all measures are being met to accommodate the needs of the ADU.

How will solar affect my accessory dwelling unit cost?

The cost of installing solar depends on how many kilowatts are necessary to supply your casita with enough energy to support the amount of consumption. Solar panel design & installation on average adds around $12-18K onto the cost of your build. This is something your contractor will put into your budget during the early stages of your discussions so you can appropriately plan. The benefit of this requirement is that the solar panels will help reduce your energy bills and make your ADU more sustainable overall.

Is there a tax credit for solar in California?

Yes, the investment tax credit (ITC), also known as the federal solar tax credit, is a 30% federal residential solar energy tax credit for solar photovoltaic (PV) installations. This means a qualified homeowner can submit for the 30% tax credit when doing their tax returns. The credit is calculated on the total installed cost of the solar system, and there is no maximum amount that can be claimed. 

What is the qualification for a solar tax credit?

• A homeowner has to own the solar system and pay federal taxes
• The solar system must be placed in service during the tax year and generate electricity for a home
• The solar system must be new and not an expansion of an existing system

How will solar affect my ADU timeline?

Solar installation and permitting, when completed during the ADU build, is carried out in parallel with the rest of the design & build process for your ADU, so no additional time will be required. The process looks very similar to the broader construction process, which includes design, permitting & installation, so your build will not be thrown off by the addition of solar.

All in all, this whole process will take between 60 and 90 days. The design process does not take much time, as it mainly involves determining how many panels are necessary to supply the whole guest home with energy. Obtaining solar permits, the next step, will take around 45 days depending on your jurisdiction. The solar permit is different from the general building permit, so the two will be submitted separately (all of that work is included in your proposal with SnapADU). Typically, the solar permit will be submitted first.

The actual install of the panels comes later in the build. The roof must be completely built before the panels can be put on. This takes about 30 days, and then must be inspected to ensure quality. Again, the entire process of solar permitting and installation is easily integrated with the rest of your granny flat build, so it is unlikely you will need to allocate any additional time for the installation. Your contractor will be sure to keep you in the loop during the entirety of the process.

How will solar affect my ADU design?

The addition of solar panels on your ADU will generally not affect the overall design of your ADU. All of our plans are designed to be solar-friendly, meaning they have a roof that will accommodate panels (a wide, flat roof). The majority of homes and accessory dwelling units have solar-friendly roofs, as it is most common to build a granny flat with a solar-ready roofline.

ADUs with more custom designs, or any irregular roof, may need to reconsider where the solar panels will be installed, as a flat roof is the best design to accommodate the panels. Any special considerations will be taken into account during the design phase, so you will know up-front whether your design will comfortably fit solar panels. Once again, solar panels generally fit in all circumstances, with the exception of very custom guest house designs.

Will my existing solar panels on the main home count towards my ADU required solar?

No, an existing solar array on your primary home – without any additions – will not be counted towards the CEC requirement for adding solar panels for new construction. However, you can expand an existing solar system on your primary house to cover the energy needs of the ADU, and such expansion may be counted towards the ADU during the permitting process.

As with any law there is some room for interpretation by those checking the plans, and each municipality interprets the solar requirement a bit differently. In some cases (even within the same jurisdiction) a plan checker may allow an existing solar array on the main home to count towards the ADU while another plan checker may not. San Marcos and Escondido will specifically not accept existing solar systems as counting towards the requirement, and we have also seen this happen in San Diego. As such, we can see inspectors moving towards universally enforcing the CEC requirements for new solar when adding an ADU.

As of December 2022, the County of San Diego Building Division confirmed the following for expanding the solar on the main home: “For an existing solar system, our building inspector will confirm the size is adequate to support the ADU during inspection. No need to designate this on the permit to expand the solar system. Please ensure the size of the system matches the energy compliance forms (i.e., CF1R).”

Could I have my own solar vendor install a system on the ADU?

Unfortunately we cannot have outside vendors work on any components of the ADU due to liability as well as logistics. Specifically, the solar (PV) system design is integrated with the ADU electrical design, connection lines to the main service panel, as well as components to the main service panel itself. While it is possible to design and install these various aspects of the electrical system independently, it has proven to be extremely problematic in our experience. So for this reason, our solar vendor who designs all of these systems also executes the installation.

Does an ADU need Title 24?

Yes, Title 24 requires that all new construction – including accessory dwelling units – must meet energy consumption thresholds. Title 24 of California state law refers to a number of regulations California has put in place to ensure energy conservation, including solar requirements.

All in all, new California ADUs must have solar panels to reduce the energy consumption of the structure being built. This requirement is carried out under Title 24 of California state law, which seeks to ensure energy is being consumed responsibly and as conservatively as possible. The process of permitting and installing solar is simple, and can be carried out alongside the ADU permitting and building process. This usually takes between 60 and 90 days. Most ADU designs will comfortably accommodate solar panels, with the exception of irregular, uncommon designs. In those cases, your contractor will be able to walk you through the alternatives available to you.