This post intends to document the various changes that have occurred in the 2019 California energy code regarding ADUs since the time it was published and took effect on January 1, 2020.
How are ADUs treated under Title 24 Code?
ADUs can be treated in two different ways – as new construction, or as an addition.
The only scenario where an ADU would be considered a newly constructed building is if it was a new structure and shared no common walls with the existing building. ADUs that fall into this category do not qualify for any exemptions under the Energy Code.
Additions are changes to a building that increase conditioned floor area and conditioned volume. Attached ADUs, as well as converted existing structures, are considered additions.
Here are a couple of common scenarios that will help with classification:
When an existing attached unconditioned structure (like a garage) is converted to an ADU, this is considered an addition. See Figure 1 below for an example.
When an ADU is built new sharing a common wall with the existing house, this is considered an addition. See Figure 2 below for an example.
When an existing detached unconditioned structure (like a garage) is converted to an ADU, this is an addition. See Figure 3 below for an example.
When an ADU is built new and is detached from the existing house, this is a newly constructed building. See Figure 4 below for an example.
More detail can be found in Section 9.3.5 of the 2019 Residential Compliance Manual.
Exemptions for Unconditioned Structure Conversions to ADUs
Exterior Wall Insulation
When an existing attached or detached unconditioned structure (like those shown in Figures 1 or 3 above) is converted to an ADU the walls can be treated as:
Wall Extensions. When an addition is built with a connection to an existing wood-framed wall, an extension to that wood-framed wall is allowed to retain the existing dimensions. This means that if the existing wall does not have continuous insulation, then the extended wall does not require continuous insulation and the existing framing size may be kept. Unfortunately, this typically will only apply to one or two walls of an addition.
Existing Wall with Siding. Similar to a wall extension, when an existing attached or detached unconditioned structure (like those shown in Figures 1 or 3 above) is converted to an ADU the walls can be treated as an “existing wall with siding". In this case, if the exterior siding (or cladding) is not being removed, and space is converted to a conditioned space, then reduced insulation requirements apply.
In either of the above scenarios, the following insulation is required:
2x4 Wall: R-15 Insulation
2x6 Wall: R-21 Insulation
This is a huge saving, as under normal circumstances, an addition or a newly constructed building would be held to the requirements in Table 150.1-A of the 2019 Residential Compliance Manual, which mandate much higher insulation values.
Take the following into consideration when reviewing Table 1 below, and compare the "typical construction" that is required with and without the ADU exemption.
Additional detail can be found in Section 9.3.5 A-D of the 2019 Residential Compliance Manual.
Photovoltaic
Solar electricity generated by photovoltaics (PV) is not required if the ADU is an addition.
PV is required for detached, newly constructed ADUs.
Smaller ADUs will require smaller PV systems, or may be exempt from the PV system requirements if they meet Exception 1 to Section 150.1(c)14:
No PV is required if the effective annual solar access is restricted to less than 80 contiguous square feet by shading from existing permanent natural or man-made barriers external to the dwelling, including but not limited to trees, hills, and adjacent structures. The effective annual solar access shall be 70 percent or greater of the output of an unshaded PV array on an annual basis.
The PV system can be installed on the ADU, or on the main house. In the event that panels are added to an existing PV system to satisfy the PV system requirements for the ADU, those additional panels must be part of the ADU’s permit application.
Note: Energy Code requirements, including the PV requirements, do not apply to mobile home ADUs, which are subject to Title 25, not Title 24.
Reference: California Energy Comission Blueprint Issue 129
HVAC
When adding an attached ADU to an existing home, the Mechanical Code does not allow return air from one dwelling unit to be discharged into another dwelling unit through a shared heating or cooling system. Systems without ducts are an option
A system serving an ADU must have its own thermostat. Heating systems must be capable of maintaining 68 °F at a point three feet above the floor and two feet from the exterior walls in habitable rooms. Heating and cooling load calculations will need to be provided per Title 24, Part 6, Section 150.0(h) to verify that any existing and/or new system is properly sized,
Any addition that adds a new dwelling unit must meet all applicable IAQ ventilation requirements of §150.0(o)1C. A detached ADU must meet all applicable IAQ ventilation requirements of Sections 150.0(o)1C. An attached ADU must also meet all requirements if the dwelling units do not share a floor or ceiling. The whole house ventilation airflow is to be based on the square footage of the new dwelling unit.
FAQs
I would like to build an ADU directly above an existing garage. Is this new ADU considered an addition?
Yes. By adding a second story to the garage, the new ADU is still attached to the garage, and that building’s conditioned floor area and conditioned volume are being increased. This is an addition.
Reference: California Energy Comission Blueprint Issue 130
When building an ADU on top of an existing detached garage, is this an addition?
Yes. Sharing a common ceiling/ floor of an existing structure is considered an addition, regardless if the existing structure is unconditioned space. Reference Figure 5 below.
Reference: California Energy Comission Blueprint Issue 133
Is it still an addition if the new ADU is built side-by-side with the existing garage?
Yes. A new ADU that shares a common wall with an existing garage is an addition. Reference Figure 6 below.
Reference: California Energy Comission Blueprint Issue 133
When a newly built ADU is attached to the existing home by a breezeway, or covered walkway, is this an addition?
No. This is a newly constructed building. It does not share a common wall or common (or adjacent) ceiling/floor. See Figure 7 below.
Reference: California Energy Comission Blueprint Issue 133
When converting existing conditioned space, like a conditioned basement, into an ADU or junior ADU, is this an addition?
No. This is an alteration. Energy Code requirements may be triggered if altering a component which is covered by the Energy Code. Some examples of alterations that are covered by the Energy Code are newly installed water heaters or mini-split HVAC systems, lighting upgrades, changes to the building envelope, etc.
Reference: California Energy Comission Blueprint Issue 133
Can an existing photovoltaic (PV) system be used to meet the PV requirement for a newly constructed detached ADU?
No. The ADU must have a newly installed PV system to meet the requirements.
Reference: California Energy Comission Blueprint Issue 134
Can new PV modules be added to the existing PV system to meet the PV requirement for a newly constructed detached ADU?
Yes. New PV modules may be added to an existing system to meet the 2019 Energy Code requirements. 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.
Reference: California Energy Comission Blueprint Issue 134
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